You sing the song when the team has come together, at the end of the meeting. All kinds of songs then might work to send them off invigorated. The Red Flag. The Internationale. Jerusalem. Edelweiss. Faith of Our Fathers. The Sash. The Marseillaise...

For starting a meeting the best team building song I know is "Why are we Waiting?" (to the tune of Come all ye Faithful). That builds team spirit splendidly among the people who've been drafted into going along, though it leaves the people running the meeting somewhat out on a limb.

Of course if you wanted a song which would help get across the message that by doing different things together people can come up with an effect that is a lot better and more interesting than they might think possible, getting them to sing a round might be quite effective. London's Burning, Frere Jacques, anything.