need to buy the base seller a beer. so you really need 30k +100 caps. everyone has to be close to the base seller and the team leader. no one can be part of another faction such as bos,vc,enclave,raiders.
I had the same problem. We added one more guy to the group and it worked fine. I saw this happen twice and both times it was solved by adding a 4th guy.
From now on, anytime you want to add another base to that faction, you can do so without any helpers along. Your leader (the base buyer) can go get additional bases solo.