You can download the expansion on PlaceholderAPI's eCloud at https://api.extendedclip.com/expansions/playerlist/

Head over to PlaceholderAPI's config file and search for the playerlist section

• playerlist:
    # Text shown when the index is out of range
    offline-text: "Offline"
    # Separator for filters such as PERMISSION, WORLD and PLACEHOLDER
    argument-separator: ','
  
    # Here's where you defined all your lists
    lists:
      # The name of this list is "staff"
      staff:
        # We only want online staff members
        type: ONLINE
        # If we're part of the list, we want to be included
        included: true
        filters:
        # Does the player have the group.staff permission? 
        - PERMISSION:group.staff
        # Can you see the player? (usually managed by vanish plugins)
        - CANSEE
      all:
        # We want all players that ever joined the server
        type: ALL
        included: true
        sorting-types:
          # Sorting players that are online first
          - PLACEHOLDER:%player_online%:yes,no
      custom:
        type: CUSTOM
        # Your custom list of elements (supports placeholders)
        input: "4, 2, 9, 5"
        # Character(s) that separates each element in the input
        separator: ", "
        # Filters are supported if the input contains player names.
        sorting-types:
          # Sorting elements starting with the lowest number (in this case: 2, 4, 5, 9)
          - LOW_TO_HIGH
  
    groups:
      myGroup: # name of the group
        gap: 2 # slots gap between each list
        lists:
          staff: # list name
            title: "Staff Members (%amount%)"
            max: 10 # will display a max of 10
            remaining: "... and %remaining% more"
          all:
            title: "All Players (%amount%)"

• %playerlist_<name>_<list|amount|#>% (see Output Types)

• ALL - Matches all players
• ONLINE - Matches online players
• OFFLINE - Matches offline players

Note: Aside from the PLACEHOLDER sorting type, when no placeholder is provided, the name of the player/element of the custom list will be used instead.

• list - Returns a list of players separated by ,
• amount - Returns amount of players matched
• <number> - Returns the player at the specified index, starts from 0.

Groups can have both a list of, well, player lists, and gaps (amount of empty slots separating each list) When adding a player list to a group, you can specify its title (text that will be shown before listing the players), a max amount of players to display, as well as a text with the amount of remaining players in case you defined a max amount.

If you're using LuckPerms (which most people do and should), you can easily check if a player has a group by checking the group. permission.
Head over to your PlaceholderAPI config.yml file and find the playerlist section, then add the following:

  playerlist:
    lists:
      my_awesome_group_list:
        type: ONLINE
        included: true
        filters:
        - PERMISSION:group.<groupName>
        - CANSEE

Now you can use %playerlist_my_awesome_group_list_amount% to get the amount of players that have your group! (don't forget to replace <groupName> by your actual group without the <>, you can also change my_awesome_group_list to whatever you want)
Note: Players with OP or the * permission will always be included, as well as people that have multiple groups.
If you instead want the player's primary group or just not include OP/* permission, or you're using another plugin aside from LuckPerms, use the PLACEHOLDER filter and check if %vault_group% is equal to your group (e.g. replace the PERMISSION:group.<groupName> line by PLACEHOLDER:%vault_group%=<groupName>)

This requires the WorldGuard expansion to be installed

  playerlist:
    lists:
      spawn:
        type: ONLINE
        included: true
        filters:
        - PLACEHOLDER:%worldguard_region_name%=spawn
        - CANSEE

  playerlist:
    groups:
      tablist: # name of the group
        gap: 1 # slots gap between each list
        lists:
          staff:
            title: "Staff Members (%amount%)"
            max: 10 # will display a max of 10
            remaining: "... and %remaining% more"
          vips:
            title: "VIPs (%amount%)"
            max: 0 # 0 = no max, redundant
          players:
            title: "Members (%amount%)"
    lists:
      staff:
        type: ONLINE
        included: true
        filters:
          - PERMISSION:group.staff
          - CANSEE
      vips:
        type: ONLINE
        included: true
        filters:
          - "!PERMISSION:group.staff" # We don't count staff members who may also have the VIP group
          - PERMISSION:group.vip
          - CANSEE
      players:
        type: ONLINE
        included: true
        filters:
          - "!PERMISSION:group.staff" # Same thing, but for both staff members & VIPs
          - "!PERMISSION:group.vip"
          - CANSEE

You can then either use %playerlist_group_tablist_amount% to get the combined amount of players of all the lists within the group, or %playerlist_group_tablist_<index>% to get a player/list title at a specific index. Can be used with TAB's layout feature for dynamic playerlists sizes!

      fixed-slots:
        - 1|%playerlist_group_tablist_0%
        - 2|%playerlist_group_tablist_1%
        - 3|%playerlist_group_tablist_2%
        - 4|%playerlist_group_tablist_3%
        - 5|%playerlist_group_tablist_4%
        - 6|%playerlist_group_tablist_5%
        - 7|%playerlist_group_tablist_6%
        - 8|%playerlist_group_tablist_7%
        - 9|%playerlist_group_tablist_8%
        - 10|%playerlist_group_tablist_9%
        - 11|%playerlist_group_tablist_10%
        - 12|%playerlist_group_tablist_11%
        - 13|%playerlist_group_tablist_12%
        - 14|%playerlist_group_tablist_13%
        - 15|%playerlist_group_tablist_14%
        - 16|%playerlist_group_tablist_15%
        - 17|%playerlist_group_tablist_16%
        - 18|%playerlist_group_tablist_17%
        - 19|%playerlist_group_tablist_18%
        - 20|%playerlist_group_tablist_19%
        - 21|%playerlist_group_tablist_20%
        - 22|%playerlist_group_tablist_21%
        - 23|%playerlist_group_tablist_22%
        - 24|%playerlist_group_tablist_23%
        - 25|%playerlist_group_tablist_24%
        - 26|%playerlist_group_tablist_25%
        - 27|%playerlist_group_tablist_26%
        - 28|%playerlist_group_tablist_27%
        - 29|%playerlist_group_tablist_28%
        - 30|%playerlist_group_tablist_29%
        - 31|%playerlist_group_tablist_30%
        - 32|%playerlist_group_tablist_31%
        - 33|%playerlist_group_tablist_32%
        - 34|%playerlist_group_tablist_33%
        - 35|%playerlist_group_tablist_34%
        - 36|%playerlist_group_tablist_35%
        - 37|%playerlist_group_tablist_36%
        - 38|%playerlist_group_tablist_37%
        - 39|%playerlist_group_tablist_38%
        - 40|%playerlist_group_tablist_39%
        - 41|%playerlist_group_tablist_40%
        - 42|%playerlist_group_tablist_41%
        - 43|%playerlist_group_tablist_42%
        - 44|%playerlist_group_tablist_43%
        - 45|%playerlist_group_tablist_44%
        - 46|%playerlist_group_tablist_45%
        - 47|%playerlist_group_tablist_46%
        - 48|%playerlist_group_tablist_47%
        - 49|%playerlist_group_tablist_48%
        - 50|%playerlist_group_tablist_49%
        - 51|%playerlist_group_tablist_50%
        - 52|%playerlist_group_tablist_51%
        - 53|%playerlist_group_tablist_52%
        - 54|%playerlist_group_tablist_53%
        - 55|%playerlist_group_tablist_54%
        - 56|%playerlist_group_tablist_55%
        - 57|%playerlist_group_tablist_56%
        - 58|%playerlist_group_tablist_57%
        - 59|%playerlist_group_tablist_58%
        - 60|%playerlist_group_tablist_59%
        - 61|%playerlist_group_tablist_60%
        - 62|%playerlist_group_tablist_61%
        - 63|%playerlist_group_tablist_62%
        - 64|%playerlist_group_tablist_63%
        - 65|%playerlist_group_tablist_64%
        - 66|%playerlist_group_tablist_65%
        - 67|%playerlist_group_tablist_66%
        - 68|%playerlist_group_tablist_67%
        - 69|%playerlist_group_tablist_68%
        - 70|%playerlist_group_tablist_69%
        - 71|%playerlist_group_tablist_70%
        - 72|%playerlist_group_tablist_71%
        - 73|%playerlist_group_tablist_72%
        - 74|%playerlist_group_tablist_73%
        - 75|%playerlist_group_tablist_74%
        - 76|%playerlist_group_tablist_75%
        - 77|%playerlist_group_tablist_76%
        - 78|%playerlist_group_tablist_77%
        - 79|%playerlist_group_tablist_78%
        - 80|%playerlist_group_tablist_79%