If they give you the name then you can make sure that the guest is well sited and provided for. If your friend did not get a wedding planner you can assist with the wedding planning: You can help :Share the cake, blow the balloon etc. You can help drive people around. On the wedding day there is always guest to move from the church location to the reception hall. If someone needs a ride be there to help out.
You can also host out-of-town guests. We all know having guests in the hotel is always a big deal. If you have a spare room you can let the couple know you are willing to host any of their guests.
Take pictures: If you have a digital camera take many pictures of the bride and groom. Make sure to send it to them after the wedding with a nice note. This is something they would love.
You can make gifts/keepsakes for the guest. Make sure you preserve samples for the couple otherwise they may never know you shared gifts to their guests. Above all rejoice with the couple