If you are fortunate enough to have a Data Layer this change may be less painful.
1- update sprocs with addition criteria 'group' (defaults to All, or not filtering)
2- at the datalayer add a function that checks the logon and determines which group to pass to SQL sprocs.
If you are not filtering at the database you can only filter at the app level (inefficient) and still requires coding changes.
You may also need to have additional table/s for which group/s have access to which projects/customers/other/?.