Can I build out Brand Showrooms for my clients?

Edited

Yes, you could build out Brand Showrooms for clients and then share the Brand Guidelines with them. They'll only be able to edit the Brand Showroom if they're an admin on your account or if they're invited to your account as a standard user and then they create the Brand Showroom. If you share the Brand Guidelines with a client who has their own account on Ethos then they'll only be able to view the Brand Showroom under their "Shared" tab.