Convert user between workspace and base collaborators

Manually add each user to each base that they need access to. If some people need interface only access, do that instead of base access. Make sure you add with the proper permissions—creator, editor, or read only. Each person will appear twice—once as a base collaborator and again as a workspace collaborator. Finally, manually remove the people from the workspace share. Do it in this order to ensure that the person does not have an interruption in access to the bases. 

Switching someone from base-only access to workspace access is even simpler, just share the workspace with the person. You can also remove the person from the direct base share to make ongoing maintenance easier, but it is not strictly necessary. 

Or, if you have the funds, upgrade to enterprise. With an enterprise account, you can have multiple workspaces, each with its own set of bases, and you can put people into groups. 

Thanks for your response @kuovonne . I tried adding a workspace collaborator to the base (and was then going to remove them from the workspace as you suggested) but the system gave me an error, stating that the user already had access to the Base.

I ended up just deleting the workspace user and re-adding as a base user. I was trying to avoid that but my team now has the desired privileges.