It doesn't feel casual, which is to say, it doesn't feel like an invitation she can decline. Not that she doesn't want to - she is cautiously interested in whatever this is about, not entirely persuaded to trust it at face value - but a healthy wariness of Leliana in general seems...healthy. It's probably not about writing more about elves again, at least, but -

"It's his space and not mine," she says, very reasonably, admittedly choosing the path to her seat that involves the least risk of crossing his. "I understand his feeling."

She finds most guests offensive, too. There's a reason, probably, that invitations to her own quarters are fairly rare for anyone that isn't Morrigan or Kieran.