Assignee group is a multi-value dimension (issues can have several assignee groups), and excepting the groups cannot be done in the aggregated member with the Except function.
The solution is to use the member of the unnecessary groups and subtract from the total in the custom measure. Assuming you have Assignee and Service Users aggregate (“Assignees and Service”), the formula could be the following:
[Measures].[Issues created]-
([Measures].[Issues created],[Assignee Group].[Assignees and Service])