I really like using email for staying in touch, also works with groups.

But besides the fact that my friends aren't really into the whole tech thing (lot of them are still using their old non-smartphone) Inlining videos, photos and threading conversations is something I really like for such a software to have.

One other idea that comes to mind is that you could create a private subreddit on reddit.com and make it closed/invite-only. Reddit doesn't ask for personally identifying data.