Most of them are easy for a serious Trekkie like yourself:
(picture with arrows found online)
But let’s do this properly.
The one you probably didn’t know
The guy in the top right was the hardest to identify: his name is Patrick Baker, owner of Farrell’s Ice-cream Parlor and a celebrity handler for Wizard World Chicago, who was working at the Con that day as a ‘celebrity handler’. Here’s what he said about the event:
I’m the photo bomber. I’m a friend of the cast and I help them out at some conventions. Levar [Burton] took this photo as I was trying to herd them towards the stage door. Since I couldn’t duck fast enough, I decided to back up and commit to the photobomb.
Pics or it didn’t happen? Here you go:
The short-changed Klingon
The small fraction of a face on the far right belongs to Michael Dorn, who played Worf in Star Trek TNG and DS9:
Reference here for Dorn’s presence if you can’t see enough of him to believe it.
The exotic half-human
The woman on the right is Marina Sirtis, who played Deanna Troi in Star Trek TNG:
The one who brought them together
The guy at the front, who gathered the others for this picture, is LeVar Burton, who played Geordi LaForge in Star Trek TNG:
The never-aging miracle
The bald guy just behind Burton to the left is Patrick Stewart, who played Jean-Luc Picard in Star trek TNG and doesn’t seem to have aged a day since:
The bespectacled Number One
The guy on the far left is Jonathan Frakes, who played William Riker in Star Trek TNG:
The venerable android
The white-haired guy just behind Stewart’s head is Brent Spiner, who played Data, Lore and Nooniæn Soong in Star Trek TNG (as well as B4 in Nemesis) and Arik Soong in Star Trek Enterprise:
The ginger doctor
The woman at the back is Gates McFadden, who played Beverley Crusher in Star Trek TNG and has kept her looks by ageing hardly more than Stewart:
The one from ages past
The square-faced man looking grumpy and confused is William Shatner, who played James Kirk in Star Trek TOS (that’s right, TOS!) and was brought through a temporal wormhole to join the TNG cast:
The guy behind Frakes’s head, and the owner of the patch of hair visible between Spiner and McFadden, are both currently unidentified. The answer from @Praxis confirms that even Patrick Baker isn’t sure who they are.
The top-left corner guy likely works for the Rosemont Theatre.
Here’s that photo again, taken right from rand al’thor‘s answer (which he took from a preexisting web article), and for completeness I’ll say that the top-right guy is celebrity handler Patrick J. Baker — also see rand al’thor’s answer for the details.
The real mystery is the guy in the top-left corner. I asked Patrick Baker directly.
@RemnantsOfPraxis: If you’re talking about the guy in white behind Frakes head, I don’t know. He may have worked for the theater.
The theatre in question is the Rosemont Theatre (where Chicago Comic Con 2014 was held).
I tried to find a staff listing for the theatre in hopes of comparing, but I couldn’t find one, and even if there were, it probably wouldn’t include non-administrative personnel.