No. Cynic or not, I'm unsure why you're assuming the entire research results made no sense! The administrator of a group should sign the "add member" messages

No. Cynic or not, I'm unsure why you're assuming the entire research results made no sense! The administrator of a group should sign the "add member" messages, and group members should only believe that a member has joined if they have such a signed message authorizing the member. You are correct in so far as this is apparently not what happens at the moment.