Your best bet is to go to the area you want to play then put out a zone chat message asking if anyone wants to quest/grind. In my experience people don't usually join in to assist with questing because it's generally boring. However, you can probably find people to grind dolmens, run public dungeons or mob grind with.

If you want to group grind without grouping up or being social, go to public dungeons (one in each zone, marked with stone arch icon) or dolmens. There will almost always be other players there, and usually people don't care if you tag along. And if you see someone grinding, put the "group up?" Emote up or do a zone chat. Maybe they'll want to run that dungeons w you.