I was there during Mardi Gras time and during the regular hours they have the parade and it's a fight for the beads...let me tell you! People will to snatch them from you if you get a hold of any...some people are vicious! lol!!
but...I was there one time and missed out on seeing a Smash Mouth concert. Bummer. But they have concerts there...I think they have two different stages set up? If I remember correctly...or at least they perform either in what used to be the "Boneyard" and then the corner across from Mel's Diner and Cafe La Bamba. But, I haven't been when they actually had the concerts and am not sure what it's like...things are pretty crowded around that time (not nearly as bad as Summertime though). Probably more crowded during the evening events too...
They have various ways. The most common is when your company books to be on the float.
If not that, there is a stand-by method. It has been changing, though. This year people were randomly picked out and offered the chance to get on the float. I got to do that on one of the last nights. It really made one family's day, as it was their last day and it was the daughter's b'day.
We were randomly asked to be on a float and had a blast tossing out beads to the crowds! It was the last night of our trip and made a real memorable ending to our first visit to Universal.