You can do it in a party. No clue if you can find it during explorations but they definitely show up in advance quests.

Yup. You have to add their gamertag. It's a bit unfortunate.

Press Enter to open up your chat and then hit the settings button at the top right. There should be a "Victory Performance Settings" tab that will allow you to change your victory pose.

It's also worth noting that you can switch between sub-palettes by opening the weapon palette switch menu and then pressing G or H.