No. You do not have to upgrade everyone in your Workspace to Pro. You can have up to 25 free users in a Workspace, and free users can create up to 25 Workflows for free.
If you have upgraded to Tango Pro, our licensing is per seat, so you can still invite a limited number of free users to your Pro Workspace. There are some limitations you will experience:
Free users won’t be able to download branded PDF’s, even if they’re in the Pro Workspace.
Pro features like Live Blur and Desktop will not be accessible to free users.
Free users can only create up to 25 Workflows.