The Core fundamentals of the game need to be built around group play.. its that simple.
Soo many MMO's add this as an after thought. As OP said. Group finders and such are poorly designed hidden away features.
I think an MMO like planetside is a good example. Group play is at the crux of the game design. You can join a group or not, but are still all working together. Another is Guild wars 2, where it is very simple to play together with someone, yet the "Find a group" feature is horrendous / extremely under utilised.
for a PvP focused system it need to at its core make the objectives Group orientated. I cant stress enough at how much UX plays a roll in people knowing how to group, why they should group, the benefits over running solo, and etc..
MMO players are inherently selfish. force/incentives for them to work together, and that’s when the magic happens. People have Fun.