Got a notification at the garrison status button next to the minimap this morning.  I guess 4 days of apexis dailies at iron horde areas made them hate me :)

Here's the wowhead guide: http://www.wowhead.com/guide=2700/garrison-invasions

I'll add that I'm not sure the score requirements are accurate since Hem and I got like 593 before the last boss and ended up with a final score of 645 or something and only got bronze.

This is one of Blizz's new no load screen instancing tech (I think), and it basically loads up a scenario for you, hence the 3 person limit.  Apparently the bags you get from gold has a chance to contain ilvl 645 gear.

So if you get an invasion, get 2 lvl 100s to help!

Edit:
Reading the comments suggest that gold rating needs 1000 pts. So maybe bronze = 500, silve = 750?