We've got a lot of options to chose from, but if you don't see the one you need in the dropdown, you can definitely create your own!
Head on over to Wedding Party, then click "Add A Person", and select "Customize Your Own Role" at the bottom of the list. Once this option is selected, you'll be able to add their role.
Add the person's name (and a description and photo as well). Don't forget to save!