Twitter began testing Circle with a small group of users earlier this month, and it now appears that it is beginning to roll out to more users (via Android Police). The feature works similarly to Instagram’s Close Friends in that it allows users to send tweets to a specific group of people rather than the entire Twitter community.
It is currently unknown how many people have access to Circle. More users (including myself) appear to be seeing the feature appear when they begin composing a tweet, but some users continue to report not having access.
