To be clear, do you mean "Team" selection??
In gmod, groups are role based and meant for, majority of the time, administrative/server commands (kick/ban/access control/etc).
Teams are intended for game/multiplayer differentiation, perhaps control what weapons they can use, their goal depending on parameters of the game mode.
Though ULX also isn't intended for team selection, and it only provides a 'team color' selection based on group assignment using UTeam in Sandbox gamemode (where teams don't normally count for any difference), there are many gamemodes and mods for them that use teams.