Can Steam players join friends on the Xbox version?

depends entirely on the game, there's no general rule. if the game has crossplay between PC and Xbox then yes, the Steam version and the Xbox version share lobbies, you usually add each other through the game's own friend system or an Xbox account linked in game. if it doesn't have crossplay, no, and the store page will say. which game? some only have it between pc stores and not with console, which is the annoying one