But are collectives allowed to exclude people? If not, how will they select appropriate workers? If you can just show up at a collective and say "I want to work

But are collectives allowed to exclude people? If not, how will they select appropriate workers? If you can just show up at a collective and say "I want to work here, and you can't exclude me or kick me out" then how will they be able to function? People will just leave collectives with poor returns and make collectives with good returns take them on.