I'm open to a polyamorous relationship and my girlfriend wants to have one with me and her ex. This would be my first polyamorous relationship and I don't really know the guy. Should I be worried about it? Possibly if he steals her away from me instead of it being a group relationship?
If your girlfriend wants you to just start dating a guy you don't really know, that's not very fair or reasonable. You cannot 'assign' or 'agree' people into relationships - it doesn't work that way. Don't date a guy just because your girlfriend wants you to.
It makes more sense for her to start dating him, and you get to know him, and see how the two of you feel about each other. There is nothing wrong with a V-shaped polyamorous relationship. Being friendly metamours is often the best way for people to relate.
If she's adamant that she only wants a closed/triad/group relationship, then she'll need to be patient and wait for you two to meet someone or grow close with someone that you also want to date.
As for your second question - no, I would not advise you to be worried about that. I can't promise you that it won't possibly happen, since no one can predict the future, but polyamory tends to make it less likely that someone will leave you for someone else, not more. Also, it's impossible for him to "steal her away" - if she leaves you to be monogamous with him, or anyone else, it would be because she made that decision herself, and you can't control her decisions. Unless he's saying and doing things that make it seem like he's trying to shift into a monogamous relationship with her and get her to break up with you, I wouldn't worry about this.
Something to be worried about, though, is that he's her ex. Why did they break up? Does that reason still exist? Are there any red flags or concerns you have about him? Getting back together with exes is not typically a great idea; so be sure you understand what his deal is, why she wants to get back together with him, and whether you want to be part of a situation that involves him.