There seem to be two schools of thought regarding how RPG campaigns are organized.
The first holds that the campaign is an extension of a particular group of friends, who get together to share each others’ company and for whom the game is a reason to do so. In this conception, “the group” is a coherent whole and “the game” exists to serve its needs and the needs and/or desires of the members of the group. Thus, scheduling, venue, and the choice of game (and the role of game master) is determined by consensus of the group.
The second holds that the campaign is an end unto itself. The individual players assemble for the purpose of playing the game, and thus the roster of players may change from session to session or more gradually over time. This does not preclude some members forming (close) friendships, but such is secondary to the primary goal of playing the game. Thus, scheduling, venue, and the choice of game is determined by the game master, and those participants who are able and willing to play, do so as they will.
This can be a problem when the participants, or a subset of the participants, thinks they are playing one sort of campaign, and others think they’re playing another. The sort of campaign should be clear to all from the outset.