We are continually trying to strike a balance between flexibility and simplicity in our UI/UX, and from our experience we’ve found that managing user permissions can quickly get complex and confusing. For this reason, permissioning is currently only supported at the Workspaces level.
Also, not to worry: you can have unlimited Workspaces with no additional charge. We count users at the Organization level, which means that you are billed for the total number of users in your Organization (past 10 users, of course), regardless of how many Workspaces they are members of. Inviting guests as Observer users (which are free and unlimited to paid subscriptions) might also be a good option for providing visibility to your work.