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: https://github.com/nextcloud/server/issues/4275).
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.