According to the rules (
http://riograndegames.com/getFile.php?id=116), up to two colonists are placed in the guest house during the Mayor phase, like any other workers assigned to any building. After that, though, at any time (start, during, or end of any other phase) they may be assigned to a different building (where they stay until the next Mayor phase, when all colonists can be freely reassigned).
I'm unsure exactly how this is implemented here, though.