Making a subadmin with some permissions

Is it somehow possible to make a group admin. How can make

  • Users (a max amount of users, lets say 25)
  • Does not have the permissions to install aps in the market
  • Have a specefic amount of disksize he can share?

No it is currently not possible. For some of your points there are open feature request on the bug tracker (e.g. group quota:

If you want to do this today: You could install different Nextclouds (for each group one). Then each admin can install apps he wants and create users. Via partitions/system quota you can control the storage they use. Users can share files between the different Nextclouds via federated sharing.