Yeah, you still can. It's pretty frustrating though. In addition to what Batabii said, the invites don't always work. First, you should only use the invite window that comes up at D0 of arcade gates (enable it in options if you disabled it). The invite thing in the social tab where you enter an email won't work. Furthermore, I can't quite tell why some of my friends, who are online and have never played SK, won't show up in that menu. In the end, I had invited 8 different friends, but never got the Steam Knight Mask, and just resorted to using my own alt accounts (since I already legitimately invited at least 5 people, I felt justified in doing this).