As a recent usher (got to wear an awesome black and white dress), after we conducted our usher duties we sat in the back of the church. I had to usher bride's elderly grandfather out at the end since he needed assistance and then hand out directions to reception. I don't think you can be with the wedding party since some of us were also closing the doors prior to the brides entrance and opening it for her to enter. I was sad to be in the back but oh well...
as far as I understand, they can stand at the back of the church as people enter and help elderly people to their seats, let guests know where not to sit (like the first two rows, etc) and also seat the mother of the bride and groom, grandmothers, etc. They can also be groomsmen as well and then join the wedding party after everyone (besides bridal party of course) has been seated.
