Twitter started testing Circle with a limited group users earlier this month, and it now appears that it is starting to roll out to additional users (via Android Police). The feature is similar to Instagram's Close Friends because it allows users to send tweets to a small number of people rather than the entire Twitter community.
At this time, it's unclear how many people have Circle access. Although it appears that more users (myself included) are now seeing the feature display when they start writing a tweet, some users still claim being unable to use it. It's still in testing, according to Twitter spokesperson Joseph Nunez. "We're still testing Twitter Circle with a group of people around the world on iOS, Android, and the Web," Nunez said. "As we continue to gather input, the feature has not yet been rolled out widely to everyone."
Update your Twitter app or go to Twitter in a web browser to see if you have access to Twitter Circle. You'll see a drop down menu at the top of the screen that says Everyone when you start typing a tweet. You can choose Twitter Circle to limit your tweet to a certain audience, or you can edit your Circle to add or delete people. Users will not be notified if you add or remove them from Twitter.
You can add up to 150 people to your Circle, regardless of whether or not they follow you. "Only those in @yourusernameTwitter's Circle can view this tweet," a message will appear at the bottom of Circle-only tweets (and vice versa). Tweeting to your Circle is similar to having a private account, only it simply restricts access to specific tweets instead of your entire profile.
Your Circle users would be unable to retweet anything you post to it, but they will be able to screenshot and download your tweets. It's also crucial to remember that even if you're tweeting to a smaller audience, Twitter's community guidelines still apply.
Source: The Verge