If you have created a topic, right now would be a good point to invite people. Navigate to People.
Click on + People to start adding people. Send invites with the users email, first name, last name, and invitation message. Once an invitation a sent, the user will receive a email with an invitation link.
As more people join organize them in groups. Name each group with a descriptive title as it relates to projects, teams, departments, etc. Clicking on Group, then +Group, you can build lists of people and access groups when assigning them to topics or briefings.
Tip: Try inviting users from a topic. This way users know what they are being invited to.