I truly admire your determination to try and do things but this is absolutely very basic user management applicable to all *nix systems. With all due respect, you are in over your head.
I’d start with reading the Digital Ocean tutorial - Initial Server Setup for the OS that you are using (you can Google it).
The other article you linked in another post (about Kolab) is a guide to installing under Ubuntu 17 … don’t. You want to use a LTS release (either 16.04 or 18.04).
You can Google things such as “How do I create a user in Ubuntu 18” … “How do I assign a Ubuntu user to a group”, etc.
I would suggest asking in the Kolab user help forum, but it seemingly is nothing but spam by a spambot … which would make me wonder if Kolab is a well maintained project personally. I quit looking at posts at the 24 hour mark, so it may all be just from today … /shrug
In conclusion, please feel free to continue asking about MiaB here but normal system admin functions are well beyond the scope of this forum.