Sorry, Experiment , but I beg to differ.
This is my sidebar right now...

Every time I try to access the Forum Group link, it brings up this...

I've double-checked my forum groups and I'm definitely part of sfborrow (literally used it last night and got pinged for using it). I don't know what's going on or why I'm not allowed to access it. Anyone else having this issue? Is this a P&B thing too? Hard to figure out in this particular case.
One thing I can think of is that perhaps that group was pinged to a thread that's part of some other forum group that you're not part of. This sort of thing has happened before, and that's usually what it ends up being.
Ffffff... I'm checking with other member of the Style-File group and the Stimulate thread. I don't even know where to find "Events & Planning."
And of course I don't know what group I'm not in because the link leads nowhere lol
Edit: it was an accident on Thomas' part. That makes sense now.
Glad you figured it out. It's been known to happen sometimes accidentally, or people forget they're posting on a private forum and tag other groups.