Confirm with your colleague that they used the unique registration URL required for registration when they created their account. If they did not, resend the URL to them and ask them to follow these steps:
- Click on the URL
- Confirm their email address, click Continue
- Confirm they are being routed into your group (they should see the name of the group on this screen), click Continue.
- They can then log in and will be properly associated with your group.
This video demonstrates the registration process: