To add a guest to a Teams channel, make sure the following steps are taken:

1. Verify that the guest is in your organization’s directory.
2. Check that the guest's email address is listed correctly in Active Directory.
3. If the guest is a verified compliant user, make sure their organization is in your verified domains list.
4. Check your settings to ensure external access for guests is enabled.
5. Ensure that the guest has accepted their invitation to your organization.
6. Finally, make sure the guest is signed in to their account and the Teams app on their device.

If you have taken all of the steps above and you are still having issues adding the guest to a Teams channel, reach out to your IT support.