I think that's a very interesting question and one I've given some though to.
First off, I would imagine this group needs to be large enough to defend itself from small bands, but not so big we would need to repopulate on our own or suffer long term isolation. I would guess that somewhere between 15 and 30 people would probably be sufficient.
Politics is always a concern, but I do firmly believe in equality and in the event that I can find like-minded people who I trust, then I would have no problem sharing equal rights with others who lived here and giving up any rights of 'ownership'. If the SHTF then we all need each other more than we need the final say over any issue and having intelligent people to help moderate group decisions can be a really good thing.
I don't think you really need leaders in a group this size, but what you do need are good mediators and people who understand the need to be reasonable and can take charge of the jobs where they have the greatest expertise. One bad apple could cause a lot of issues, so democracy is the only way to deal with things fairly imo.
My comrade here is a 27 year old orphan who had no prospects or family in London. He worked at the same company and I offered him free room and board here in exchange for some hard work and good company. Obviously he is investing his time in this, so I then gave him a decent % of the property in my will and promised at least 30% of the gain in price if we ever decide to sell up and move on.
I would never tell him what he can and can't do here and he is master of his own fate - we're actually going now to pick up a puppy that he wants (I have 2 cats and don't really want a dog, but I would never stop him getting one just because it's 'my house').
Power corrupts and absolute power corrupts absolutely

. I hate cetralised power and I'm pretty convinced that both capitalism and communism might actually work quite well if you removed the corrupting influence of centralisation. I have no desire to be 'the man'; I don't deserve all that I have and would happily share it with my fellow man.
Hippy Love