At present a user can use only 1 operator in a filter.

What's happening now:
Reporter: Specify User -> john smith

What I'd like to see:
Create new filter
Reporter: Specify User -> john smith AND sally jones AND sam stone
or
Create new filter
Reporter: Specify Group -> jira-developers AND specify User -> NOT john smith (who's a member of jira-developers)

This functionality could be extending to Assignee and other custom fields.