Hello,
Thanks for the additional information. Not sure if you are aware but we already have a screen for cases. If you have the CRM or Full Blown Edition you would be able to access it. Also with this screen you already have a dropdown that refers to the Method Users so you can select who the cases are assigned to. This would take care of the 1st part you asked about. If you like you can just copy this screen and modify it. Check it out and see if that would accomplish what you're looking for.
Note: Since there is a table called Cases already you wouldn't be able to create a table with the same name.
To learn more about how to setup up users/roles please take a look at Webinar 1: http://www.methodintegration.com/cs/blogs/methodblog/archive/2009/07/13/webinar-1-getting-started-with-method.aspx
The last step of the puzzle is to filter your grid. To do this there are a few things you have to put in place. You would want to make sure you add a field to your table to capture the group role of the person entering the invoice. You can either do this by adding actions to your save button or you enter the group name on screen. You would also what to add an action to capture the role when the screen loads and put it into a share result which you would then use to filter the grid. So basically you would then compare the role of the active user logged in and the role outlined in the case.
Hope this helps.