In order to solve this inconvenience I've created Xteams! a web-app where teams managers can give scores to players in the management panel. Players of the group can't access this panel but can see the list of players, mark which of them arrived to the game and create teams easily.
At the time of writing, the algorithm behind the teams allocation is pretty simple. It takes all of the available players, and the number of teams to create, and try to find teams with equal or close to equal strength (sum of the players scores).