I’ve had issues on two occasions where I was using gw2lfg.com and received an invite. Both times I would get the option to “Join In…” but when I would click it I would load into the new instance but no portraits would render. I could see the icons for the party members on the mini-map but that’s it. I finally noticed something today though… One of my party members was on the Kodash [DE] server (a EU server) while I’m on Northern Shiverpeak (a NA server).
I think a simple fix would be to either completely separate the NA and EU servers, so you can’t invite one from the other, or two just allow cross-server parties. As it stands right now, it looks like you can message and invite people regardless of what server they are on, but you can’t actually get to the same instance.