Indian wedding ceremonies are preceeded by the Baraat. This is the outdoor procession, just before the ceremony, in which the groom rides in on a horse for up to an hour. You'll want to be in something you can move around in!
Women: Wear a Lehenga, Anarkali, or Gown. If you want to wear a sari, this is your time to do it since you will be seated for a good portion of it and saris are the most traditional Indian outfit. Do NOT wear red (the bride will be wearing red) or black.
Men: Wear your fanciest kurta. These will often have a bit more embroidery or gold work on them. Perhaps they have more of a collar or an embellished stole. Again, don’t wear red or black. The groom will likely wear a beige “sherwani” (similar to a kurta) with red and gold highlights. However, that does not need to stop you from wearing beige/red/gold as well as those are the most common colors for kurtas.