Party also kicks you out of the party if you logout,

Is it possible to keep you in the party unless you leave or get kicked

Thanks tai! Inviting someone who's already in a party now tells you 'Already have joined a party.' You also get a message if they decline or the party is full. Logging out no longer takes you out of your party: log back in on the same character and you're back in it. One gap is left: if the leader stays logged out for a couple of minutes the party still breaks up. Leadership will pass to another member instead in an upcoming update.

Follow-up: a party no longer breaks up when its leader logs off. After about a minute and a half, leadership passes to the highest-level member still online (one on the leader's map first), everyone sees who took over, and the old leader rejoins as a member when they log back in. If the whole party is offline, it waits 30 minutes for someone to return.