Since it's all in one bill and the location will probably be a Washington location, you should all factor into your meal-bill tax before you even considering tip or no tip.
From the Vic dinners my friends and I have hosted in years past, Tax had always been an issue since the majority were coming from Oregon- some people would bring way too much for tax and some would bring way too little, or the money that was intended for a tip was invested into the tax. Everything did work out, of course.
It's lots of fun to hang with people you don't normally do for an hour or two
If you get the chance and can go, I'd highly suggest going. Also if you know the restaurant in advance, you can usually go in and request to have a copy of their menu to look over prices, or it will be online. Most locations don't go over 10-15$ for a main-course, and most appetizers are 3-7$. Places like the Dragon Fish has a variety of prices, while places like the Cheesecake Factory (both in Seattle) are on the higher end and could top 20$. So, you
might want to look into sharing plates.