how do i join THEIR party?

i can only click on their name and add them my party, how do i join their team instead of getting them to join mine?

If your party has less players then the other party then you automatically join their party. If you are asking about you inviting 1 person and wanting to join them with that person as the leader, they have to invite you.

Click on the party leader, then click the green "+". Just like you would to invite a member to join YOUR party.