How particular you are about the people you share with will depend on how much interaction, cooperation, and group decision making will take place during the sharing arrangement, as well as how easy it is to leave if things don't work out.
In larger sharing groups, you might not be very concerned about whom you share with. If you join a 15 member tool-sharing group, for example, you may not ever meet some of the other members. The main qualifications for members might be that they are adults, live in a certain town or neighborhood, and take good care of the group's tools.
But if you share a house with someone, you'll undoubtedly want to be much pickier. (See Chapter 2 for qualities you might want to look for in your sharing partners.) Ultimately, most sharing arrangements come down to finding someone you can trust to respect your needs and belongings, and for whom you can do the same.