Can I mask the full user group and only show the ones that are assigned within a closed project?
Welcome to the Community Forum @MariAnne_Vanella!
You can create hidden teams and private projects. Then when you invite users at the project level, they will be added as Members with access to specific projects in your team. They will only be able to see the members of the project they were added.
Please note private projects are a paid feature, you can find more information about project permissions here.
I hope this helps!