That is going to be true with certain discussions ... but you need not be a member of a group to post to that specific forum (if that is how it was set up)
Only way you can avoid this is to make the submission of a group pending approval so that it is a group wherein discussions would only be of interest to those in the group with similar interests.
I wanted to bring back my TKD Parents group I had on the former site but I see now it is a topic on the forum so I did not start it up again