Seats are checked twice: when you send an invite, and again when somebody accepts it.
At invite time
You're using all 3 seats on your plan. Add a seat or upgrade to invite teammates.
Pending invites count towards the total while they are outstanding, which is deliberate: it prevents sending an invite that could never be accepted.
At accept time
The workspace is at its seat limit (3). The owner can add a seat from Billing, then you can join.
Somebody claimed the last free seat between the invite going out and this person clicking the link. The invite itself is still valid, so once a seat frees up the same link works.
Options before upgrading
- Revoke stale invites. Invites sent months ago to people who never joined are holding seats. Revoke them in Profile then Workspace and the count drops immediately.
- Remove departed teammates. Their lists and exports stay with the workspace.
- Check whether they need a seat at all. Somebody who only needs the data does not need a login. Export the CSV and send it, or push it to the CRM they already use.
- Then upgrade. Pro is 3 seats, Scale is 10.
Seat changes take effect the moment they are made. Adding a seat by upgrading applies immediately and is prorated, so a mid-month hire does not have to wait for a renewal.