IsInGroup (Method)

IsInGroup( integer groupId )
Returns boolean memberOfGroup
Description: Checks if the Player is in the group whose groupId is inputted as an argument.
Member of: Player

LocalScript vs Script

Whenever you call IsInGroup using a normal script, you will not get the most up-to-date information. If a player leaves a group while they are in the game, IsInGroup will still think they are in that group until they leave. However, this does not happen in a LocalScript instance. A LocalScript will always return the most up-to-date information about the player's groups.

Example
game.Players.PlayerAdded:connect(function(newPlayer) -- Runs when a new player enters the game
   if newPlayer:IsInGroup(7) then                    -- Checks if the player is in group 7 (Roblox fan club)
      print "Player is in the Roblox Fan club!"      -- Runs if the player is in group 7
   end
end)