Any of the dozens of "rounds" sound much better with the added harmony of the two additional parts than solo. That could illustrate your point....we are better together than apart....ie try to clap with one hand..

Also, on a teamwork theame you might consider "I Would Like to Teach the World to Sing." From a militaristic angle you could consider "Give Me Some Men Who are Stout Hearted Men and I'll Soon Give You 10,000 More." From a nationalist view you could consider "This Is My Country." You could also consider any of several audience participation songs, ie "The Noise Song"......'Half th greatest moises in the whole wide world, comes from me, comes from me and you....1.give me a clap, give me a stomp, give me a shout 2. snap ----cough----whistle, 3. dog--horse---laugh 4. kiss ....grunt.....some quiet.....5. give me the end, give me the end. Or, on a religous note, "Will the circle be unbroken" "Helping Hands" Or from a child's view, "We're all together again," or "There's a Meeting Here Tonight."