Remove experience gains from playing in LAN, and let it happen.
Why?
1.) It's nice to be able to play with just friends.
2.) If you're worried about software pirates being able to play multiplayer without a valid serial code, throw that excuse out the window, because they already are.
3.) More and more people are starting to hack, let us play with a group of people we know and trust, without being forced to deal with scores of hacking jackasses.
4.) Competitive reasons, both in-game and in business. Most FPS games with a focus on multiplayer come with LAN support, why not Crysis 2?
/END RANT