Google+/features/Circle

"Circles" were a feature of Google+ which made these things possible, from a given user's perspective:

• I can categorize people I follow
• I can choose a particular category of people to "read" at any given time
• I can choose a particular category of people to be allowed to view something I post
• I can choose a particular category of people to notify of something I post

In other words:

• by viewing a circle, you could see a list of users and a stream of content from just those users
  • This is similar to "lists" on GlitchSoc.
• those users could be notified whenever you posted something to the circle
  • This is a bit like a hashtag that someone else creates and can uniquely curate (good) but from which you can't unsubscribe (not so good).
• those users could be given exclusive access to whatever you posted to the circle

What they don't let you do:

• categorize incoming posts (so you don't have to include or exclude everything someone lets you see)
• let people categorize themselves (so they can determine which of my posts they want to see)

Admin duties for a circle could not be shared, to the best of my recollection.

See also communities, for a G+ feature which addressed those missing features.