Well, DBT group is easy, all you say in the beginning is that you're attending a support group for people who have similar struggles or experiences (depending on what exactly you've told her so far)... You don't have to mention the acronym.
As for when? Well that's unfortunately up to you to decide.
Most people would argue the earlier the better, but I would argue that it depends on a few variables, especially since we're talking a PD, there's a difference between axis 1 and axis 2 diagnoses for a reason.
Me personally?
It's easy to blurt out that I have Bipolar, I don't care much for any preconceived notions or stigmas others may have about an illness I suffer from.
But besides my bf, members here and treatment providers, not a soul knows I have a BPD dx too.
Mainly because it affects my bf directly, I vent or seek advice here, and treatment providers obviously need to help me and its simply nobody else's damn business.
But the other side of that coin is the fact that since its a personality disorder others might and probably will a) treat me like a pariah and b) not realize how manageable it can be, kind of like a life sentence of some kind.
Soooo long story short.
Tell her when it feels right, when it can, will or does affect her, but don't wait until you're walking down the aisle.
But give her enough of a chance to get to know you without any labels attached. That way she already has a good idea of who you are, minus the possible complications that accompany your personality.
I had the perfect opening with my bf, I was having a classic bpd push pull meltdown over the phone and he didn't understand how or why he was seemingly just making things worse.
So I sent him an email with a BPD fact sheet titled "Its more than bipolar"
I was honestly expecting to be dumped that night.
Instead he drove all the way to my house to hug me and tell me we're gonna be alright.
Best of luck to you