Regarding Rhidian's question/experiment: We tested this a while ago. If you remove a char from the party in any way, the char is no longer playable. The player that controled the char gets unasigned from it and is left with his other playable chars if he has any. Every playable char has to be in the party. Kinda sad, as it makes a lot of fun mod ideas impossible.
So it would be quite easy to create a DM mod that allows a player to spawn monsters, take control of characters and place items etc. on the fly. Just the other players will see which character the DM is currently controling, will see him on the mini map and will share vision with the DM etc. So it's kind of pointless for now.
Speaking of which, since the game doesn't identify players, only the host can assign specific players to characters and every playable character is always asigned to a player, there is no way to remember who was the DM and make the DM character unplayable when the DM is not in the server.
And regarding Demonata08's question/experiment: You can set two players hostile. Actually we do this in the divine arena mod. The debug log will go rampage if you do so, first stating "this should never be done" and then spams errors for like 15 seconds - but it recovers from it and works after. Anyways, hostile players again still will be in the same party, see each other on the minimap, share vision etcetc.
I really hope larian will make a patch to allow for multiple parties anytime soon - amongst a lot of other things. Anyways for me that there's only one party and every player has to be in it is one of the biggest issues. That and that we cannot assign players to chars scriptingwise and prevent the host from assigning players makes it really hard to create anything but main-campaign-style stories with the editor.
