Hi @RichardJTelford,
You haven't missed anything: RStudio Cloud only supports sorting projects by project name or by date created at present. I'll add this to our list of feature requests, but I unfortunately can't promise anything about when it might be available.
Since you mention this:
The latter would help me find out who is actively working on the project and who has given up.
Have you seen the space usage feature? It's not as streamlined as what I think you're after, but you can check out activity in your space by clicking on the Usage icon in the header:
which will take you to a page that segments overall activity by space member. From there, you can to click on each name to see that member's activity segmented by project. If you click on one of the projects in that list, you can see when it was active.
Hope that helps,
Mel