Why recruit just one person when you have room for two or three? Everyone wants to survive, and everyone in that group gets along with yours, so why wouldn't everyone come along? It streamlines the game, keeps people from having to pull their hair out deciding who to bring, and removes the unnecessary padding of plucking one survivor from each Enclave. Not to mention it gives you a better reason to become allied with an Enclave. Lastly, it will allow people to quickly prepare new communities for use in things like Multiplayer. It's nice to have a few extra lives stored up for the more hectic sessions.
If an Enclave is only Friendly and not Allied, then you should only be able to take one survivor as usual, and the rest will run off. If there is concern about the game becoming too easy due to players simply recruiting new followers en masse or storing several Allies for a rainy day, then make any extra recruits cost influence.