Search for "Codestream: Invite" in Atom's command palette (accessible via the Packages menu) and you'll be able to see a list of everyone currently on the team. You'll also be able to invite new members to the team, and re-invite anyone who hasn't yet joined and needs an extra little nudge.
Your teammates can also add themselves to the team. Just have them open one of your team's repositories, install CodeStream in their IDE, and then sign up. CodeStream will recognize, by virtue of a match on the URL of the remote and the hash of the first commit ID, that the repository is already connected to a team on CodeStream and the developer will be added to the team.