Here's what I found:
App settings >groups > only me, or customize, but this effects all groups.
App settings >groups >additional permissions> uncheck 'streams' - still affects all groups.
I don't see a way to block only a single group from view... it's all or nothing.
Personally, I don't think most people will notice or care that much about what groups you join. The chances of someone seeing you join a certain group, doing research on it and concluding you have something to do directly with it is slim to none. If anyone says something of that nature, they probably already had suspicions.
What you could do is not just join and aspy group, but join a bunch of 'disorder' groups like that around the same time. Changes the odds, you know?
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When I allow it to be
There's no control over me
I have my fears
But they do not have me