First, thanks for your answer.
Maybe it's an option to build in several steps first, which we can use right now. Those are maybe easier than what you describe here above. Those steps can be:
1. Projectstatus (project metadata) - preferaby self-adjustable names - or maybe simpler at first 'active' or 'not-active'
People don't see the 'not-active' projects in their project overview
2. Possibility to delete more than one person at a time from the project. (In the future account portal possible?)
We have projects which contains over a 100 users who we now have to delete one by one if the project is ready.
Those two steps help already and then the the third step: the need to store the data somewhere so that it can be retrieved, can be developed parallel.