Note: If you are in an Active Directory environment this is not the solution for you! You would use AD to create and modify all permission settings.
First, enable or ensure LDAP is on. If not enable LDAP and enter a new administrator password.
Scroll down to users and groups in the same menu. Click Add Group. It is often cleaner to add groups first so that a default group with the same name as the user doesn't get created
After groups click add User. You can assign users to multiple groups
Continue till finished.
To modify a user or group just select the options
To modify share level permissions for a share go to the Pool, Share, then expand the arrows to the appropriate share and select User-group access
If guest is enabled for the share you will have to go to "Protocols" under the OPTIONS for that share and deselect "Guest" and select "Users with password"