Twitter is testing a new function that allows you to have a conversation with just a few people. The announcement was made via their official twitter handle. The function is still in the testing phase and is only available to only a few people globally.
To use the feature, Compose your tweet and mention the people you want to talk to, then tap on the permission stings at the bottom left side. Here, you will select who you would like to reply to the message; the options are, everybody, people you follow or people you mention only. If you select the people you have mentioned, other users can view and even like your conversation but they will not be able to reply to it.
The invited participants will also be able to tag and reply to each other on the tweet. Previously, anybody could join in a conversation whether or not they were mentioned or invited. This is part of Twitter’s strategy to promote responsible social media use and prevent unwarranted comments on private conversations.