I would say the most important thing to establish at the start is whether she sees her relationship with her man as the only long-term life-partnership she ever wants, in which case all other relationships would be "secondary" (sounds bad, but can be a totally lovely way to share intimacy with someone even though you're not sharing a life), or whether she's open to the idea that she could some day have two life-partners (aka "primaries"). I think knowing what the possibilities are will probably make you feel less adrift.
Regardless of her answer (and she may just reject the terminology entirely, as some people do, so I would recommend using something like the phrasings I employed above rather than the shorthand terms of primary/secondary, since the concepts have value as disscusion points whether or not you like the terms), chances are very very good that you're going to be secondary at this point in the relationship -- her long-term partner will come first. After all, terms aside, who would meet someone new and immediately treat them just the same as their life partner?
Being in a secondary relationship with someone who has a primary when you don't have one of your own can be disconcerting and leave a person prone to insecurity. You may feel like you don't know where you stand. Remember, if you're confused about how she sees something, or what's "allowed" you can always just ask. If your gf is loving and supportive and you're actively examining your feelings and talking as you go along, it will just get easier and easier. I highly recommend everything on this website, but the essays under this heading may be particularly useful: http://www.morethantwo.com/polyconfigurations.html
Eventually you will want to figure out if you want another partner (or two or more!) of your own or, if she can give you enough to make it worthwhile, if you only want to be with her. Many people make such a relationship work, where one person is polyamorous and the other is monogamous (poly/mono). It's up to you to make that call but it by no means has to be made right away (nor is it immutable for that matter)... some people find having multiple new relationships in the establishment-phase to be too much to handle anyway, so it may not be an issue for some time regardless.
Getting to know her man at some point will probably be a big help. Assuming he's a cool guy, seeing him as a person rather than as a mystery or a rival will help head off jealousy. You guys could even potentially develop a great friendship some day -- after all, since you share the same woman, you'll always have a topic of conversation ("does she do that annoying huffy thing with you when she's losing an argument too?" "wanna go in on a birthday gift together this year?").
Good luck!! Hope you stick around and keep us updated with your story.