Forming a party
Press the yellow group button on a player's row in the Shell Phone to invite them. You see "Squad created, you are now a squad leader" and "Invited RELLsin to your squad". The invited player gets "RELLsin has invited you to a squad", and the invite turns into an envelope. After they accept, "RELLsin has joined your squad" appears.
Party Members panel
The panel reads Party Members [2/5] and has the hotkey [P]. It lists each member with a coloured diamond marker and a number beside the name. It has five marker slots, with empty slots shown for missing members. The numbers change during a fight and look like health values, but the game does not label them.
Friendly damage
A developer said that in a party you can still damage each other, but with reduced damage. He warned players to be careful when teaming on people and on bosses.
In boss fights
The Aegon fight was shown with a two player party. The Progression panel with hotkey [L] sits above the party panel.