This is a tough one. You would think that a little common sense would go into play here, if they know your son has CF. You would hope that they would not come if it meant exposing Nate to possible colds.
If you have all of their email addresses, maybe you could send an email to all of them & don't hide the addresses, so they know they're not the only one receiving the email. Just ask nicely if they will refrain from bringing sick kids to the party because Nate catches things so easily.
The other way to look at it is, Nate's bound to get colds, and many of them, throughout his childhood--whether it be at Church, play groups, at school, from a shopping cart, etc. I'm not helping here, am I? lol
I don't remember my parents ever shielding my sister & I from groups, colds, etc. Maybe they just weren't smart enough to think of it, or maybe there wasn't such an emphasis on germs back in the 60's-70's either??
Good luck & let us know what you decide. <img src="i/expressions/face-icon-small-smile.gif" border="0">