My girlfriend recently broke up with one of her partners she's been dating for a long time and is not feeling well because of their reaction. I am thinking about contacting the others to met and organize a surprise to cheer her up, like spending a week with everyone near the sea.
The problem is that we never talked to each others (except one time at Pride, to say a quick hello and shake hands). I fear she would feel uncomfortable, but at the same time it would be great for her to see the ones she loves instead of playing video games all day to not think about it. What would you advise me to do?
This is a sweet and adorable idea, but you're right that it might be a big, uncomfortable thing to spring on her as a "surprise." My recommendation is to plan something less intense - a weeklong vacation is a pretty big deal, but you can still set something nice up for her. I don't know your gender or the genders of her other partners, but since she's a woman, she's probably used to the expectation that she do a lot of the logistics and organizing for fun things, so doing that for her will be lovely.
I don't think it would be out of line to reach out to her other partners on Facebook or wherever, to explain that she's feeling down and you want to put together something nice for her. It might not be wise to throw everyone together - she may feel anxious or responsible for managing the relationships if all of her partners are at an event together - so you could consider planning something individual for her to do with her various partners.
Going to a local spa, seeing a band she likes in concert, going to a movie, booking a nice dinner, a picnic at a pretty nature spot - choose something you think she'd like. The key is that, after you make sure the time and date and plan works for her, you and her partners put everything together. You call and make the reservations; you book the tickets; you find a campground; you pack the picnic; whatever. All she has to do is show up and enjoy.
Then you say "Elbreth is taking you out to a nice dinner on Friday - she'll be here to pick you up at 6:30." Or "Let's go on a picnic this Saturday, I've got everything sorted out - you just need comfy shoes." You could also expand this effort to include her close friends, not just her partners. I would lose my mind with joy if someone close to me said "hey, I arranged for a babysitter and did the calendar-wrangling with your friends, Galadriel and Arwen are taking you to dinner and that art show in Rivendell on Friday."
When it comes to the larger thing, like a trip or a get-together with all her partners, my advice is to check in with her about what she would like, when works for her, etc. Once she's told you what she wants to do, who she wants to be there, and the dates that work for her, it's on you and her other partners to coordinate and organize. That's the real gift; the "surprise" bit is less relevant. Of course, you know your girlfriend - if she really loves surprises, keep some details secret and do what would make her feel most cared for!