You can accept yourself (as a male), convince Alistair/Loghain to do so, or refuse and sacrifice a Warden's life. This is what Morrigan's Dark Ritual consists of.A mage Warden who enters the Fade to confront the Desire Demon controlling Connor but then converses with her can be rewarded with sex, in exchange for the Demon being allowed to (or pretending to) leave.Or, if you romanced and hardened Leliana, she will automatically invite herself along, and you can then invite Zevran.
If female, you can invite a hardened Alistair to participate. Their father abandoned his family to go to the surface, after Kalah refused to come with him to seek a better life from Orzammar. You can even achieve it with every single one of them at the same time, though most of them will be a strictly platonic adoration.
