She doesn't have to specifically find out, the next time she see's Elliot, she'd notice he's not as cheerful as the last time she saw him so she'd ask whatsup, Elliot can at the very least say he was hoping he could get everyone together an do something on Saturday (he doesn't have to say anything about it being a party) and he feels bad about Susan's response.
Dan himself said he's not exactly sure when and how Elliot would have found out about Susan's experience in France, he mentioned the fact that Susan had told Sarah and Grace, but it's also possible he heard it from Nanase and/or Ellen cus we know Nanase told Ellen about Susan's angst induced awakening, she could have told Ellen about France as well since she was involved too, Nanase has just as much right to choose who to tell as Susan does and Susan didn't ask for Nanase's permission to tell Sarah and Grace so I don't see why Nanase would need permission from Susan to tell someone.
Ashley's been pretty resistant so far, she opted for seeing a movie in a public place rather that in private where there could be a chance of Elliot transforming for fun or something. That'd be a big reason to avoid attending a get-together with people she recently learned has experienced magic in some form, she's trying to avoid temptation, probably not out of fear of giving into it, but out of the possibility of having a breakdown in front of everyone.
Considering Tedd's main reason for wanting to make magic public was so that it could be used to help people, "Gung ho" might still be apt.