About 2 weeks before your Meetup, you'll receive an email from OI with a lot of the logistics details. Check-in begins at 3pm and takes place in a production building next to Hard Rock Cafe on CityWalk (there's a little path on the right-hand side of the restaurant, aka to the north, I think it used to be the Blue Man Group building?). There will be plenty of big signs/flags and staff members directing you to the exact spot. If you have daytime admission, you can check in at the US park next to the Minion Mayhem ride (there will be signs and staff there as well to help guide you).
You have the choice to begin with whatever park you want - if you want to watch the Holiday Parade, start with Universal Studios. Some people recommend starting with watching the shows like the Bourne Stuntacular or the Horror Makeup, which don't run during the Meetup.
We're going the 15th-16th which won't have the parade, so we're probably going to check the ride times on the app at 5pm and decide from there whether to hit US or IOA first. We're probably going to grab a snack from the check-in bites and hit a few lower wait rides in that 5-7pm timeframe. A lot of people in the FB group recommend doing one park per night, but I think we're going to play it by ear a bit - our top priorities are Hagrid's, Velocicoaster, Gringotts, and Mummy. Everything should be a walk on (or close to it) after 7pm, except Hagrid's, so I'm trying to chill and not overplan (despite my anxiety dreams where I somehow don't have time to ride a single ride haha... ha...).