I'm interested can a PS Vita player join a PS4 party? (And yes it works) but as you know there's the new party system. Actually, a Vita player can join a party from PS4 only the new system is there
Yes, you can do this by adding and adding to the party.
But the new party system is on the PS $ with news and such
Oh, you have to create a chatt group and add that player. Then go to the typical party icon, press on create. Then you have to select a group. Choose the one you have created and then you have a party.