The U21 Team Manager chooses if he wants a certain player to be part of the U21 Team. If you think your player deserves to be part of the U21 Team, try sending a mail to your U21 Manager. Your team does not get any benefit from having a player on a national team, but that player does get a benefit. From what I know a player on a national team gets experience.
Here is the link.
http://www.buzzerbeater.com/BBWeb/rules.aspx?nav=National...Quoting
National team managers can recruit or dismiss players from a national team at their leisure, so long as these players are currently active
Having your player on a national team will not affect his player ratings or weekly minutes in any way. He will garner experience, but cannot suffer an injury that will carry on after the game.
Hope this helps. :)