How to get write accesss for external storge for SOME users

Nextcloud version 18.0.3:
Operating system and version Ubuntu 19.10
Apache 2.4.25:
PHP version 7.3

I have set up a local external storage, this works fine. In the config dialog I find an option to make the complete mount read only. For the average of the users this is ok, but I have few users who should be able to update files on this mount (which would be impossible when I make the complete mount read only).
How can this be solved? Do I need a second mount (ugly)!
Is there some where an option to set an admin ?
Do I need an further plugin to see what I miss ?

Thanks a lot!
Michael