If they’re on the app, there’s a ring around their avatar.
Anytime you see a blue or other colored ring around an avatar in your contact list – whether you’re looking at your chats, choosing people to send a Moment, Check-In or Thread Starter to, or inviting people from your “My People” screen – it’s an indication that they’re already on the app.
To connect with them, send them a request to connect. You can do that a few different ways.
Learn how to connect with people who are already on the app here.