How does Exp. Share work? If I have 5 other Pokemon in my team, do each of them get less experience versus having only 1 Pokemon on my team? Is it beneficial to have my party full when using Exp. Share or does it matter?

I'm trying to complete my Pokedex and am missing the Fossil Pokemon evolutions. I didn't know if I should have one in my party at a time or all of them in there while training.