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

Solution 1:

I don't know if it's possible, I had Leliana joining but there was a black screen so it was not so much fun...

Maybe Morrigan must loves you for joining. Howewer you are losing nothing. Really :P

Solution 2:

I do not believe it is possible. According to http://dragonage.wikia.com/wiki/Isabela, as a male character you can only bring Leliana (and only if she is hardened) and/or Zevran to join in the fun. As a female you can bring Alistair (if hardened), Leliana (if hardened), and/or Zevran.

The Last Doctor is correct about the event - there is nothing to see other than a fade-to-black screen when you choose to have a bit of fun on her ship...