Yeah, that may problematic.
Here are some threads that might be useful:
https://vborg.vbsupport.ru/showthread.php?t=196613
https://vborg.vbsupport.ru/showthread.php?t=112718
https://vborg.vbsupport.ru/showthread.php?t=107342
https://vborg.vbsupport.ru/showthread.php?t=163063
https://vborg.vbsupport.ru/showthread.php?t=187992
I think the problem is more about privacy implementation than permissions, am I right?
I will add more if I find something I will think it may be useful for this purpose.
By the way, I would like to add that most wanted activity stream feature making it really useful is filtering. It makes much more sense when user can limit his activity stream to what are doing his/her friends otherwise it's just what's up on the board..
Looking very promising so far!