What Christos suggests is a great idea. My group was getting bombed with bogus member join attempts after we had tried setting our group so that anyone could read our group's messages and/or join.
When that proved problematic we changed our settings to require moderator approval of new member applicants PLUS we modified our welcome message to include the requirement that a new member applicant provide us with a brief Intro Biog as a condition of membership approval. An email address to respond to was included –along with the aviso that if we had not received an Intro Biog from them within five days their membership request would be declined.
We also set up a bump-reminder message indicating that we have not yet received the member applicant's Intro Biog which we send out the day after the initial membership request.
So far for us this arrangement has worked well and no bogus memberships have accidentally slipped by us.