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
![Smiley :)](https://www.kumoricon.org/forums/Smileys/default/smiley.gif)
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.