1. In your Outreach project, click on your name at the top-right and select Admin Console.
2. Click Add Member.
3. Type in a user's email address and choose whether they should be an Account Admin or an Account Contributor.
4. Click Send Invite.
5. Go to your Outreach project.
6. Go to the Deal Team tab.
7. Click Add Team Members in the top right.
8. Click in the Add Members field.
9. Start typing the user's name.
10. Select the user.
11. Click Add Members.

If they haven't registered their name won't appear, but their email address is available for selection. When they register they can access your project.