I've been thinking about doing meetups to meet women so I'd be interested in advice for this too. I've seen it posted before on ROK or other blogs so you're probably better off looking for advice on lines to use there. But practically, I can think of a few things off of the top of my head.
You need to put the guy in his place. You're too polite to people being rude to you. Mimic what he did. It worked against you, right? You seemed to say that all he had to do was direct his attention on her and exclude focus on you. Do the same. Cut him out of the conversation, place your back to him, talk about an obscure topic that's personal to you that he has no idea how to talk about. For example, I fucking hate it when you're in a group conversation and someone whips out their phone, shows someone a photo or a joke meme, and then doesn't show you because you have no context for following the conversation. I'm a pretty good artist and I have pics of my drawings on my phone. You were next to her, so you could have easily shown her something like that, and excluded him from the conversation for a short period. If you're awkward about asking a girl for her number in front of someone else, then hand her your phone so she can put the number in without him seeing it. It'll be super awkward for him to ask for her number right after you. And it displays dominance.
If you're at a meetup, and you're working the room this wasn't your only opportunity to get her number. This isn't a date. Ask her for her number within 15 minutes of talking to her, at the height of a story about what you like to do, or at the height of a joke. You don't have to end your convo with asking for her number before moving onto the next girl. It'll either be awkward or it'll seem like you're a player to her as soon as she sees you chatting up the next girl. You want to be natural like this is day game. And lastly, your job isn't to prevent others like this guy from getting her number, because frankly it'd be nearly impossible to do so. Instead, you should be focused on standing out so she'll remember you.
And if you really are cut out from the conversation, then when you excuse yourself to talk to others just say "I had a really fun time talking with you, let me get your number now in case I don't run into you later in the night. I'll let you know the next time I'm going to one of these things." You have nothing to lose by doing so but everything to lose by not trying. Assume the sale. I highly doubt she'd refuse to give you her number, give you a fake number, or not respond when you message her. At worst she'll just be "busy" when you ask her out. And you still have the opportunity to come back to her in the night.
You need to put the guy in his place. You're too polite to people being rude to you. Mimic what he did. It worked against you, right? You seemed to say that all he had to do was direct his attention on her and exclude focus on you. Do the same. Cut him out of the conversation, place your back to him, talk about an obscure topic that's personal to you that he has no idea how to talk about. For example, I fucking hate it when you're in a group conversation and someone whips out their phone, shows someone a photo or a joke meme, and then doesn't show you because you have no context for following the conversation. I'm a pretty good artist and I have pics of my drawings on my phone. You were next to her, so you could have easily shown her something like that, and excluded him from the conversation for a short period. If you're awkward about asking a girl for her number in front of someone else, then hand her your phone so she can put the number in without him seeing it. It'll be super awkward for him to ask for her number right after you. And it displays dominance.
If you're at a meetup, and you're working the room this wasn't your only opportunity to get her number. This isn't a date. Ask her for her number within 15 minutes of talking to her, at the height of a story about what you like to do, or at the height of a joke. You don't have to end your convo with asking for her number before moving onto the next girl. It'll either be awkward or it'll seem like you're a player to her as soon as she sees you chatting up the next girl. You want to be natural like this is day game. And lastly, your job isn't to prevent others like this guy from getting her number, because frankly it'd be nearly impossible to do so. Instead, you should be focused on standing out so she'll remember you.
And if you really are cut out from the conversation, then when you excuse yourself to talk to others just say "I had a really fun time talking with you, let me get your number now in case I don't run into you later in the night. I'll let you know the next time I'm going to one of these things." You have nothing to lose by doing so but everything to lose by not trying. Assume the sale. I highly doubt she'd refuse to give you her number, give you a fake number, or not respond when you message her. At worst she'll just be "busy" when you ask her out. And you still have the opportunity to come back to her in the night.