D&D 5E DM campaign disputes.

[S'mon]

While there are variations in table etiquette from group to group I (and DMs I've played under) have always discussed the addition of new players with the group before doing so.

I've always seen adding new players, and if necessary removing players, as within the GM's prerogative.
Of course it may be good to take advice from the players too.

I can recall one recent group where the players saw it as within their prerogative to add new players
whenever the numbers dipped. As GM I went along with it, but found it a bit odd.

Usually: players ask GM if their friend/partner can join; GM tells players when a new player is joining.