if you use a domainname with a lenght below 15 chars, you sould be able to create a users and groups. I was testing very long names of domains. If I use a domain name blow 15 chars every thing is fine.
There was an issue about shared folders, but this should be solved with nethserver-ibays-3.0.1 by @giacomo
Look at this : Users unable to write on Shared Folder
Is your machine joined to the domain? If not, the credentials for login are workgoup\user + password AFAIK.
Did you check Allow write permission to owning group in edit shared folder? And you can try to add the user to ACL with read and write permission.