(I hope. Retreat to make them all want something first, if necessary.)
You might think that making them want it really really much is the trick to conflict. But while it is useful, it is secondary. The first thing you have check is that they are not compatible. If Jillian wants to be the greatest thief in the world, Prince John can ensure she doesn't raid the royal treasury by pointing out that coming with him and stealing from the Necromancer. If Jillian is oath-bound to recover the Red Star of her family, Prince John can persuade her to come with him to the Necromancer and then announce that he is bestowing on her a genuine crown jewel to reward her. It's when Jillian is oath-bound to recover it, and Prince John needs it for the spell that is keeping the Necromancer out of the realm, which only works if it remains in his possession -- that's where the conflict arises.
Harder with heroes than with villains, since their desires are more reasonable and lack the desire to push other people around. Of course, by the same token, that makes it easier to get the heroes to join up, perhaps with some compromise and sacrifice on both parts, and the villainous factions to slaughter or otherwise alienate each other, so it's not all bad.