Float rider Signup link
https://www.universalorlando.com/RSVP/UOAP-Mardi-Gras/Sign-Up.aspx
General notes:
They are doing the parade every day, even non-concert nights. Concert Nights will be the hardest to get.
The meeting spot for float riders is in front of A Day in the Park with Barney in Kidzone. Usually they start meeting 2 hours before parade time and stop about 45minutes to an hour before the parade, if at the end of the window they still have open spots (usually no shows or people who sign up for more slots than they use) they will let anyone near Barney who asks on. (If the weather isn't great the odds of "standbys" getting on are high, concert nights or good weather, not so much)
I think I recall a handful of people informally lining up for "standby" last year, but we had a slot so we didn't pay much attention, a few people waiting did get on though. You get assigned one of the floats by the wristband you get and then you have a spot in the barney courtyard to hang out in until your group gets, the King Gator is the largest and holds the most people (we were on it last year), they were the first to load up since it was way in the back (but quic. They'll walk you backstage and give you the outfits (just slides on top) and up you go. Tons of beads and you'll wear your arm out trying to keep up.
This video around 6 minutes in gives you an idea how its like
If you don't have the annual pass yet you won't have your AP# since it hasn't been issued.