Arena Filter is an addon which adds class and role filtering options to the premade arena group finder.
After selecting Find a Group in Group Finder > Premade Groups > Arenas, you should see a Filters button to the right of the search field. Clicking this button will reveal a list of checkboxes for classes and roles to be included in the search results. Results shown only include those which contain a member matching at least one of the checked items.
Your filter preferences are saved between sessions, but are unique per character.