Improve private groups security & administration

## Better groups security
### Privacy by obscurity is not sufficient for some users, especially in education

The privacy of a group can be too easily breached by anyone sharing the URL for the group. Users need a more secure way to form a group, both in terms of including and excluding other users--a group admin should be able to kick someone out of a group.

### Group formation based on a set of usernames and/or emails

### From users:

>I had my first workshop at Kalamazoo where I shared Hypothes.is with faculty members. I figured you had experience with this and might have perspective. Of the people who attended, one of the interesting “sticking points” (and one I didn’t anticipate) was the issue of moderation. Basically, they were anxious that anyone could invite anyone to join a group, no one could moderate anyone else’s annotations, and no one could be removed from a group by someone else.

>I am a TA in an online course and I was hoping to comment students' posts on the LMS discussion pages. I created the group but I couldn't find a way to invite students with Hypothesis accounts. It asks me to send the link to them to share the group but I'd like to be able to type their names and select their accounts so an invitation is automatically sent to them. Is there a way?

No files, tests, or entry points are identified in the issue. The requested work spans group formation, invitations, membership removal, and moderation; clarify the intended behavior and locate the existing group administration flow before defining completion criteria.

Written by the indexing model from the issue text.