Can't think of a parade that actually allows guests to be ON the floats. They do select families to serve as honorary Grand Marshall for the parade, but this process is completely random. At park opening, when there are dozens of CMs roaming the Main Street area, one of them will be charged with finding the day's parade GM. You can't find this person on your own or ask to be the GM.
Some of the parades will involve children from the audience. Characters that are on foot will sometimes take children by the hand and dance around for a few moments. If this is what you're looking for, the best advice I can give is to make sure you are sitting in the first row along the street. This could mean staking-out your place an hour or two ahead of time, but that's the price you have to pay.