At the moment I have attempted to use:
local playerName = game.Players:GetPlayerByUserId(19497313)
which works just fine, as long as the player is in the server.
Is there a way that I can get the player's name from their user Id, without them being in the server?

2 comments
Thank you, but is there a way to use functions such as 'IsInGroup', without the player being in the server?
No, because :IsInGroup() is called on a player object, which you can't get if the player isn't in the game. Please accept the answer if it helped