The St. Patrick's parade gig is a function of the grounds committee within the Games structure, so they assume they have your contact info already. The sign-in sheet is a record of who shows up that morning so they can credit a work day to count toward park and entrance tickets.
Being there the whole week of the games earns multiple pairs of tickets. The Monday cleanup day after counts.

So,if you want to be on a different committee, volunteers@smhg.org . As noted above, Sat. evening April 7th there will be an all hands on
deck volunteer meeting to kick off this year's work. I'm sure you can just go to that and say, "Here I am, and here's how I think I fit".
They'll tell you where to go. Let me know if you want to do that, and I'll run it by my committee head to verify veracity of above info.