To create a new group click New Group ...
Who within your Organisation is going to be using Workbooks? Make a list of who they are and give some thought to which areas of Workbooks you want them to be able to use.
When creating a new User you can specify:
Users can be assigned records by using the "Assigned To" field within the different record form layouts; please note however that if this user does not have a valid license allocated to them, it will not be possible to assign records to them.