Of course there's a reason he received an invitation. There's not much that goes on in Skyhold that Vivienne doesn't hear about sooner or later. Either from people directly gossiping, or from voices carrying up from the Great Hall into her balcony. You'd be amazed how amplified a "hushed whisper" can grow with so much space to expand into. Regardless of how she found out, she knows he's a mage they rescued. Therefore, he is now of great importance to her, as she takes her self-appointed responsibilities to the well-being of mages most seriously.

"There's no mistake, my dear, I assure you. Please do have a seat, I've been so looking forward to meeting you."

She waves a hand at her tea service where there's a tasty spread of sweet and savory munchies set out. It's an invitation for him to help himself.