Sorry for the delayed response on this. Did you ever figure it out? If not (or for others with the same question), the section on Limits in the guide might be helpful:
Each account is allocated one private space, with up to 10 members and 25 projects. You can submit a request to the RStudio Cloud team for more capacity if you hit one of these space limits, and we will do our best accomodate you.
If this sounds like what you're encountering: you should see a Request More Members button near the capacity message. Clicking that button (even if you've not reached your limits yet), should prompt you to provide some additional details about what you need.
Hope that helps!