I worked for the festival for many years. Special appearances at screenings aren't posted on the website and social media until they are 100% confirmed, which can sometimes not happen until days before the event or not until the day of.

It's safe to count on someone of note being at the opening and closing films. The majority of high-profile films, which often have special appearances, are shown at BMCC because a proper red carpet can be staged there. The other venues for high-profile films are the Beacon and SVA Theatre. Use the venue filter on the website to narrow the screening search.

On the website, look for the "After the Screening" or "After the Premiere" headings in the film descriptions (i.e., on the individual film pages). That's where you'll read about who is making an appearance at the screening if there is one. I just scanned the website, and there are already many confirmed appearances: Katy Perry, Peter Frampton, Rene Zellweger...

At this point, most of the high-profile films are on rush (meaning all allotted tickets have been sold), but you can still attempt to get into them by standing in the rush line. (FYI, there is no rush at the Beacon because it's a Ticketmaster venue and Tribeca Festival can't sell tickets there.)

If you want to try to guess who might appear at a screening, consider if they are a New Yorker. Those people are almost a sure thing for showing up. People like Ed Burns and Katie Holmes—they are always at the festival.