Try whispering them first. Most people are put off by an invite that has no message attached to it and may not realize it's coming from the person in the same area as them (I've gotten invites from planets away for no reason at all).
This pretty much sums it up for me. Say hello. Actually invite me. I hate when I just get a random pop up. I don't even read them, honestly.
Try this, "Hey, are you working on [quest name]?" And when they respond, try, "Me too. Want to work on it together?".
That's all it would take in most cases. I'd be more likely to team up longer if there is some conversation once we're grouped too. Nothing crazy, just a reason to be in a group. I can finish quests solo all day long. But, I can't do them AND get a good banter going while playing by myself. Well, I can, but then people call me crazy.