My goodness, another example why *no* "advice" in a forum like this can be relied on. We start with question on how to place a lien on a house and end up with facts like this. I've know my best friend for (depending on how you count) 35 years. I trust him with my life. If we were to go into a million dollar deal together--I'd make sure we wrote it up. To not do this in a situation like here is stupid beyond belief.
What is the situation we find ourselve in now? Who knows? Realistically we have a partnership.
Can a tennant in common live in the house? Yes. That's one of the rights of property ownership, possession. That's what property law tells us. But, what about a partner's fiduciary responsibility? We're not really co-owners, but in business together. Not selling in order to live there does not seem to meet the business needs. Further, if we are a partnership, the "individuals" are not really owners in the sense envisioned by Section 121 (regarding the exclusion of capital gains on sale of personal residence).
See an attorney. Try to establish some facts and work out the agreement with your partners. Failure to do so now will cause substantial problems later. Because of the nebulous nature of the relationship, people are going to start thinking. I think some of the thinking will lead to some super-genius tax fraud ideas. Do you want to be a co-conspirator?