I have recently read Creating a Life Together: Practical Tools to Grow Ecovillages and Intentional Communities, which is a book containing experience and advice for people wanting to build a community. The book is about ecological communities, which may differ in some aspects from the rationalist ones, but I believe most things are valid generally.
Some points I remember:
Do not overestimate people's commitment, no matter what they say. When the moment comes to actually put down the large amounts of money, don't be surprised if most of them suddenly change their minds.
Do your research in advance -- how much the project will cost, what kinds of documents and permissions you will need, and whether your plan is actually legal. (Ask people already living in similar communities. Actually, visit them for a few days, to get a near-mode experience. All of you.)
Good fences make good neighbors. Whatever were your original agreements, expect people to change their minds later and to remember something different than you do. Then you will need a paper record.
For any kind of group decisions, you need very precise rules for (1) who is and who isn't a member, how to become one and how to stop being one; and (2) what happens in case of prolonged disagrement. Outside view says that "we will do everything by consensus" is magical thinking predictably leading to a disaster.
It helps to identify your vision, and describe it in "vision documents" as clearly as possible. You might be surprised that people who previously seemed to agree with non-specific details, will suddenly find things they object against. (Better to find it now than after you have all moved.) Also, this will be helpful in future to explain your community to potential new members.
It is a bad idea to introduce power imbalances, such as "the rich members volunteer to intially pay for the poorer ones" or "someone can lend their unused private building to the community", because that can make later intra-group negotiations really unpleasant (e.g. when you have a vote about something the rich members have a strong opinion about, and they end up in the minority). If there is a need to lend money between members, do it completely officially, so that the fact that "X owes money to Y" cannot be used as a leverage against X.
It is probably a good idea to have together some lessons on communication skills. You need to be able to talk about sensitive topics where you disagree, without it making you feel disconnected. But you also need to hold each other accountable for things you agreed upon.
Filter people for emotional maturity. Seriously. Some people can cause insane amounts of unnecessary drama. And that applies not just for founders; you should also agree on some selection process for new members in the future. Also, newcomers should become provisional members first, participate in the community life and contribute some work, before they become full members. (Good interview questions: how have you supported yourself financially in the past? describe your long-term relationshops, school and work experience.)
There are different options: You can buy or rent several houses or flats for individual members of families, and then one extra place which will be common. Or you can buy a piece of land with several houses. Or a piece of land without houses, and build them. Or you could buy an office building or an abandoned factory, and then rebuild it. Members can own their places; or you could together create a legal entity that owns everything, and all members rent it from that entity. That entity may be able to take a loan.
Have a debate about what is your position on:
Have monthly meetings, with an agenda and a facilitator; only members can vote; take notes and archive them. (The logs will be useful to show to new members in the future.) Specifically, keep records about agreed upon tasks and dates. To make sure everyone is trivially involved even before you buy something, require a symbolic financial contribution from the members (remember, only those can vote), e.g. $100 for joining, and $10 as a monthly fee. Keep financial records.
How to create positive emotional bonds: talk about your life; cook and eat together. (Once in a while.)
Additionally: if only one member seems enthusiastic about thinking/planning/enforcing this kind of stuff that is a very bad sign. In such a situation when that person burns out the community slowly dies.
Rationalists like to live in group houses. We are also as a subculture moving more and more into a child-having phase of our lives. These things don't cooperate super well - I live in a four bedroom house because we like having roommates and guests, but if we have three kids and don't make them share we will in a few years have no spare rooms at all. This is frustrating in part because amenable roommates are incredibly useful as alloparents if you value things like "going to the bathroom unaccompanied" and "eating food without being screamed at", neither of which are reasonable "get a friend to drive for ten minutes to spell me" situations. Meanwhile there are also people we like living around who don't want to cohabit with a small child, which is completely reasonable, small children are not for everyone.
For this and other complaints ("househunting sucks", "I can't drive and need private space but want friends accessible", whatever) the ideal solution seems to be somewhere along the spectrum between "a street with a lot of rationalists living on it" (no rationalist-friendly entity controls all those houses and it's easy for minor fluctuations to wreck the intentional community thing) and "a dorm" (sorta hard to get access to those once you're out of college, usually not enough kitchens or space for adult life). There's a name for a thing halfway between those, at least in German - "baugruppe" - buuuuut this would require community or sympathetic-individual control of a space and the money to convert it if it's not already baugruppe-shaped.
Maybe if I complain about this in public a millionaire will step forward or we'll be able to come up with a coherent enough vision to crowdfund it or something. I think there is easily enough demand for a couple of ten-to-twenty-adult baugruppen (one in the east bay and one in the south bay) or even more/larger, if the structures materialized. Here are some bulleted lists.
Desiderata:
Obstacles:
Please share this wherever rationalists may be looking; it's definitely the sort of thing better done with more eyes on it.