sac on Guineas throat

That sounds really odd, can you get a pic of it? Possibly a predator (owl) attack maybe... are there any visible wounds? Are there any puncture marks or anything?

I'd cage her, feed her well, give her some electrolytes and vitamins in her water and let her rest a few days. I'd also provide her with a heat lamp for a little warmth so if it is an infection she has a better chance of her body being able to fight it off. She may be more comfortable if you keep her cage near the flock.

If it's an abscess you may need to drain it, but without seeing it I can't say for sure that it's an abscess or just swelling. Please post a pic if you can.

 Hope she doesn't get any worse 

That's ok, that's a great pic. Very clear! (Exactly what I was hoping to see!)

Poor Gracie, looks like maybe some blood/fluid has pooled there a because of a wound. I think I can see a puncture wound on her face next to her ear, possibly a tallon or tooth/bite mark from some nasty predator. She may have even bashed into something during the day and that's why she did not return to the coop that night too, hard to say. I've never seen a tick on a Guinea before, or what it does to them, but the swelling/fluid could be from the tick... was it low on her neck/throat area? Of course edema always tends to pool at the lowest point it can. It doesn't look infected to me at this point, but it may turn into that, so I'd keep an eye on it. Does it feel hot at all? When my Guineas get some kind of injury they usually move around a lot slower and kind of depressed for a few days. Guineas are pretty resilient tho, and good at babying themselves until healed. She looks exceptionally healthy, so her immune system should be nice and strong. I'd still cage her like I suggested, keep an eye on the sack to make sure it doesn't get worse or turn into a nasty infection, plus watch for any other symptoms. As long as she is eating, drinking and breathing ok she should heal herself with a little rest. If she will let you, I'd clean the wound on the side of her head and then I'd put some plain neosporin on it too. And as silly as it sounds, a warm compress on her throat area may help dissipate the fluid/blood. If it continues to get bigger she may need some antibiotics... but hopefully it's already as worse as it's going to get.

Good luck, give us a progress report  when you have time.

BTW, BEAUTIFUL HEN!!! 

I use Poultry Electrolytes with added vitamins (a powder that I mix into their water, Merrick's - Blue Ribbon, I ordered it online), but Tractor supply usually stocks Save-A-Chick electrolyte and vitamin supplement packets (which is basically the same thing). A lot of people use regular Poly-vi-sol or some other brand of liquid other baby vitamins (given a few drops at a time orally), and some even use Vionate (pet vitamins) added to their feed, but I prefer the liquid. You can use plain Pedialyte for kids or even a 50/50 mix of Gatorade and water for extra electrolytes if you can't find an all in one product/combo of vitamins and electrolytes.