I have a idea.

How about a sort of wharf, where villagers from different tribes (Not from different user) can come to your tribe?

You need to pay Tech Points to build raft to send your villagers to different tribes and "Recruit" some new villagers
(It might solve the "Thinning the tribe" problem)

You then have to wait for few hours (Or a day?) for the raft to come back.

Old villagers and childrens cannot travel by raft.