I do understand this is not possible right now, espeially given there are only 5 companions available in single-player mode at the moment. But I do hope that as Larian adds more companions to the roster, the party size in later EA versions (or even just the completed game) can be bumped up to 5 or 6 members. Even if this is not implemented in multi-player mode, or even if it is left as an option that can be chosen at the start of the campaign.