User-->UserGroup<--Group
    |
   userComputer
    |
  Computer

Here is more.

We open an account with  a computer supplier to each of our groups{fiction
writers, technology writer etc}. The supplier assigns a number to each group.
We receive a computer from the supplier with computer number as assign it to
one user within a group. A user can move from group to another but belongs to
one group at a time. A computer can be assigned to a different user. The
computer sends the supplier hourly usage by the user.

The computer supplier sends a bill containing several pages.
1)Hourly usage of each day per row by a user in a group using a computer.
  GroupId UserName ComputerId Date hoursUsed rate tax total
2)Computer usage summary. Hours used per month for each computer. Summarises
each computer use per month
     GroupId UserName ComputerId hoursUsed rate tax total
3)Group summary : summarises monthly usage per group
    GroupId hours rate total

Thanks

Since Computers and users are entities, you will need a table for each
of them.  Computer Sessions will reference a user, a computer and a
start-end time pair for usage.  UserGroups are a relationship among
users, so that table will have a group id, reference to the users and
start-end time pair for their group assignment.