User account licensing (logging in with your Redgate ID when activating), and by extension the invitation function, doesn't work with SQL Monitor and SQL Backup because they are assigned to a server rather than an individual user.
If a user has been assigned to the license, it won't be possible to activate because the server will think it's already been activated. To activate you will need to remove the current activation/invite in the customer portal.
When you activate the license it will show in the portal as Not logged in. If you want to remove it from the current server and activate it on a new one you will need to remove the activation in the portal to free it up first, then activate on the new server.
You will need to contact the license owner/admin if you can't see the license in your account.
NOTE: This also applies to SmartAssembly.