A workable open-battle system. 10v10 or so.
Talk to an NPC, it'd go something like this.
Murder [1337]: Hi
Open Battle: Hello, Murder. How can I help you today?
Murder [1337]: Open Battle
Open Battle: How many people do have to participate in this event today?
Murder [1337]: 20
Open Battle: 20 people, that means 10v10. Is this correct?
Murder [1337]: yes
Open Battle: Okay, who is the team captain of the first team?
Murder [1337]: <name>
Open Battle: <name> is the team captain of the first team?
Murder [1337]: yes
Open Battle: Please state the other 9 people on his team.
Murder [1337]: <name>, <name>, etc
Open Battle: So you want <list of names here>? Is this list correct?
Murder [1337]: yes
Open Battle: Who is the team captain for the second team?
Murder [1337]: <name>
Open Battle: <name> is the team captain of the second team?
Murder [1337]: yes
Open Battle: Please state the other 9 people on his team.
Murder [1337]: <name>, <name>, etc
Open Battle: So you want <list of names here>? Is this list correct?
Murder [1337]: yes
and you get tp'd to a battlefield, sounds a little complex to be scripted, might be a bit too advanced for an OT, but it's just a suggestion xP
Murder [1337]: Hi
Open Battle: Hello, Murder. How can I help you today?
Murder [1337]: Open Battle
Open Battle: How many people do have to participate in this event today?
Murder [1337]: 19
Open Battle: Sorry, I'm not able to proceed, there isn't enough players for even teams. Find or kick someone to fix this issue.