Get A Spokesman
Have someone come to town who wants to speak to a spokesman for the area. You could use a few different things, such as a merchant wanting to speak to someone about setting up trade, or a leader of a nearby tribe who just saw these Elves and wants to negotiate with them, or even a bad guy coming in to issue threats.
No matter what you use, the NPC in question should make it clear that talking to an entire committee is too cumbersome or beneath him (depending on who it is). He is only interested in speaking to a representative who can talk for the entire PC settlement. If they want to deal with the NPC at all, the PCs will have to pick someone to speak for them.
From there, you can let the situation organically grow and see if that turns into a leadership position. If it does, your job is done. If not...
Force The Issue
If you can't coax them, you can act more forcefully. You can try this a couple of different ways:
- Have random attacks start happening with increasing frequency. See if the PCs wind up with a defacto leader organizing them in combat (assuming your LARP has combat, you weren't specific on that). If they do, then you're done.
- If they don't organize themselves, have someone come in from a group that's also being attacked by whatever is attacking them and want to team up to fight against them. That person is going to want to talk to a leader.
- If that doesn't work either, simply have an authority figure come in and appoint someone. Maybe the land they moved into actually belongs to some kingdom but is generally unused. When the king hears some people moved in, he wants them under his authority so he appoints an official of the crown, who becomes the leader.
But Do You Really Want To?
Is this actually a problem for the PCs, or is it just a problem for you? I mean, if they enjoy doing things by committee and the disagreements that come from that, why do you want to force them out of it?
In my experience LARPing, leaders would appear naturally in groups when they were wanted/needed. If one never did, it was usually because the people involved were pretty content not having one, and trying to impose a leader on them was a great way to create resentment. That can be fine itself, if you want them to try to undermine that leader. But it doesn't sound like you want to create more infighting.
In the worst case scenario, this can cross boundaries and turn into out of character hostility. I've seen it happen: the people running a game pick a PC as leader who the other PCs don't think deserves it, and that leadership position comes with some kind of in game power. The next thing you know, rumors are flying about that person being the favorite of someone that runs the game, and that's why they were chosen. That's the kind of stuff that can lead to people quitting a LARP, even if there's no truth to it at all. On this stuff, perception is reality.
It's really best to just run the game and let the leadership issue sort itself out amongst the players. If they start failing to complete plot lines due to disorganization, you can point out that the problem was disorganization and that leadership would help them, but you should leave the actual appointing of leaders up to them whenever possible. When it's the group deciding to follow someone, the group is a lot more likely to accept it.
In the end, they're really the only ones who can make a leadership position work. It doesn't matter how much you want there to be a leader if there is nobody they're willing to follow.