If you want to @mention a person, you will be limited to only the people in the current conversation. If you would like to @mention a person that is not currently part of a conversation, you will first need to invite them to that conversation.
There are two ways to @mention a person. One way is by quoting a previous message. When you quote another user’s message, it will automatically @mention the owner of the original message. To learn more, visit Managing your messages in the mobile app
The other method is by manually @mentioning a person in your message. While composing a message, you can @mention a person by following these steps: