Earlier quoted context omitted.
What about a tab for each Path share group indicating which friends you share photos with? If you want to share photos with "work: medium-good friends" you can and insure that the "work: ok friends" or "family: mom's side" don't get the same post. I feel like the hard problem is not to keep groups separate, but to re-factor existing infrastructure to support separation and making it 'easy' for users.
I feel the hardest part is making it easy for users. Facebook has offered lists and groups for a long time, and it's fairly easy to post something to just a specific list. However, I found that the difficult part is the housekeeping of these different groups. Even if you take the time to create lists for your different social groups, these groups will evolve over time, because of new friendships, fights or whatever;…
things coming and going should be a feature not a flaw of the way people interact online...