So we've been trying to contact IOI 2015 for a couple of days now, about the invitation letters and stuff so we can start working on the visa, but they aren't responding.
Do you know anyway of contacting them other than their emails which are listed on their site?
It's very urgent so any help would be appreciated.
Based on comments on CF, it looks like misof is amongst IOI organizers (he may not be the right person to ask these stuff, but maybe he knows more contact points). You can try messaging him. (He can also be found on Quora, so I guess you can also message him there)
I believe it's much better to contact IC rather than ISC in this case.
Maybe you should try calling them (+7 7272 64 43 44, from IOI website) if it is urgent :P
I can't help you but I can confirm that IOI 2015 organizers are currently active. I got the invitation letter 2 days ago. Calm down!