Yep, I'm going with the majority here in saying "talk to your gf first," and I think joyfulgirl gave excellent reasons.
What I get out of my bf's other relationships includes: radiation of the sexual energy his other relationships generate; a special relationship with the other woman, if we like each other (it's kind of like a sister-in-law in some ways, with some obvious differences); better trust and communication than I've had in any other relationship. It's good to know he'll never cheat on me or lie to me about his interest in other women.
The "she can see other people too" angle is a little trickier. That might not be something she wants, or something you want. If the idea of seeing other people doesn't appeal to her, don't push that side of it; just let her know it's open in the future if she changes her mind. I haven't dated anyone else since I got involved with my bf, but I enjoy the freedom of knowing I can if I really connect with somebody.
If you're not okay with her (or your ex) seeing other guys, don't pretend you are. But be aware that that's going to make the arrangement a much tougher sell, and try to get to the bottom of your resistance to it. As a matter of basic self-respect, I'd never be in a poly relationship where I didn't have the freedom to pursue other people, and I think most people would feel the same, but if you can't stand the idea of either of them with someone else, you need to confront and deal with that now rather than saying what you're supposed to say, "Sure, baby, you can have other partners too" and then blowing up when it actually happens.