In Dragon Age, how can you get Morrigan to come with you with Isabela in the Pearl

http://dragonage.wikia.com/wiki/Experience gives a lot of info on experience in Dragon Age. Characters are given experience for their individual actions, and experience is not shared between party members.

Actions that give experience:

• Killing an enemy (only the final blow counts)
• Completing quests
• Picking a lock (if successful)
• Disarming a trap (if successful)
• Acquiring a new codex entry
• Donating to your allies

So, to answer the questions in order based on the info in the wiki page:

• Experience is distributed to individuals, not among party members, who perform one of the above tasks
• Dead characters do not gain experience because they do not perform any of the above actions; however, their level is automatically adjusted to one level below the Warden's once you return to camp if they are two or more levels below his level
• Unused party members do not gain experience; rather, their level is adjusted to be one below the Warden's under the same conditions as dead characters
• The character who actually kills the enemy is the only one who gets experience.

The trick to the fight is to use the litany every time he's going to convert a mage into an abomination.

I did that and the fight went fine - I don't remember it being particularly difficult.

If I remember correctly, Wynne shouts out each time you need to use the litany, so if you've killed her, this could be a bit trickier, but I think both Uldred and the mage glow white when he's about to convert them