Free Workspaces in Trello are now limited to 10 collaborators, which includes both members and guests.
The collaborator limit applies to the entire Workspace, so while there may be fewer than 10 members on the board you're viewing, there may still be more boards in the Workspace with more members. If that's the case, the Workspace admin will need to either remove collaborators or upgrade the Workspace. If you click on the "Share" button on that board and view the list of board members, you should see "Workspace admin" underneath the name of anyone who is an admin and can make the necessary changes.
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