Screenshots of what you're doing.

Nice! I was hoping someone would create something like this.

The algorithm works quite well already: One player is added to one of the two teams in each step. Also each team has a current "mean elo" value, which allows to pick the "best" player for the team from the pool. The algorithm starts with assigning the best player (highest elo) to one team, then players are added to the two teams alternatively. If the current team's mean elo is lower than the other one's, the best player is picked; if it's the other case, the worst player is picked. At the end, all players which are not registered yet (i.e. the bot doesn't know their Xonstat #id) will be randomly assigned to the teams.

I hope that this will result in balanced teams, but it needs to be tested with more people once it is fully implemented

Here's an example of how the teams will end up if we have six players in the pool, with elo values of [ 100, 250, 500, 500, 750, 1000 ] (yes, these numbers are not realistic ):

	1000 -> Team A	(best player to A in first step)	[mean elos: A=1000 > B=0]
	750 -> Team B	(best remaining player to B due to A>B)	[mean elos: A=1000 > B=750]
	100 -> Team A	(worst remaining player to A due to A>b)	[mean elos: A=550 < B=750]
	250 -> Team B	(worst remaining player to B due to A<B)	[mean elos: A=550 > B=500]
	500 -> Team A	(worst remaining player to A due to A>B)	[mean elos: A=533 > B=500]
	500 -> Team B	(best remaining player to B due to A>B)	[mean elos: A=533 > B=500]

In the end, we have two teams with mean elos of 533 and 500 in this case. I don't know if this algorithm gives the best result; if anyone has another suggestion, let's hear it! =)

Another idea which goes into the same direction is choosing the team captains by elos. Simply picking the two best players is not the best way to go imho, since then always the same people will be captains. A suggestion from asyyy was to randomly pick one player as captain, then pick the other one with the lowest elo difference. In the example from above, the captains would then be (1000, 750), (750, 500), (500, 500), or (250, 100), while choices like (1000, 100) are not possible.