If your companion leaves at the end of Act 1 on the boat at -1 attitude, there's a way to keep them in your party until you can raise it into positive.
Open up a second instance of the game by using the .exe in the /bin folder. Load your single player campaign in one instance, and enable direct connections from the menu. In the second instance, do a direct connection into the first.
Now, in the first instance, assign the second instance to control the Companion. Now when you get on the Lady Vengeance, the companion won't be taken out of your party. You'll have to keep the multiplayer connection open until you find a chance to raise attitude again, but this seems to work.
