We get the occasional potential spammer "wanting to join," i.e., hack the group. The "click here to reject" reply in the e-mail from does the job for us. But we are a "secret" group with very limited membership and a strong affinity base rather than a general interest group, so our experiences may differ from yours. I don't think there's any way to stop intruders from probing, but that's how we stop them once they have probed.?
What I'm curious about is how they even know we exist if we are indeed a "secret" group. I'd love to hear from the GMF moderators on that; it's not as though our group name is an everyday phrase as it is written in our name. Where do they get it from?