Phantasy Star Online 2 New Genesis

Pretty easy achievement, just join a party with other players in New Genesis.

You can get it by either inviting someone to a party or accepting an invite.

First you need to progress a bit into the story, pretty straightforward start, You're looking for the quest for finishing the first cocoon, finish that one and return to the command room.

Actually adding someone can be done through quick menu, use the back/view/2 squares button to access it.
You can do this with a friend through the "friend list".
Or with random players with "Nearby Characters", select player > party > invite to party, and hope they accept.
I found that people were pretty accepting if you just invite them when grinding PSE burst in combat areas.
Or just shotgun invite people randomly if you're desperate...

Likewise accepting an invite is done through the quick menu as well, in "party commands".