When our group first started this was what we perceived as a problem but it turned into a blessing. We sat down and talked about families and friends that may have been preppers. Found a lot of us were thinking about the same people. Once that was established one of us would sit down with a prospect and talk generally about the condition of America and our local area. If that talk turned ugly then that person was crossed off the list. If possible additional talks happened and the person(s) was asked to a meeting.
This may sound strict and formal but it is what worked for our group. There are sixteen persons/families in our group and we are loosely part of a state and regional web of preppers.