Two steps:
1. Login and click "Invite someone" in the right column under "People". Enter and select persons github id.
2. It will then give you the option to "Invite Username to some teams" at which point you simply check off which teams you want to add them to then click "Send Invitation"
Alternatively:
1. Get the persons github id (not their email)
2. Navigate to the repository you would like to add the user to
3. Click "Settings" in the right column (not the gearbox settings along the top)
4. Click Collaborators long the left column
5. Select the repository name
6. Where it reads "Invite or add users to team" add the persons github id
7. An invitation will then be e-mailed.
Please let me know how this worked for you!