Originally Posted by
Rivin
The invasion is the last part before The Iron Wolf becomes available. If you haven't done it, and neither Grimjaw or Durotan is offering it, it sounds like that is the part preventing you from continuing. Submitting a ticket couldn't hurt.
- - - Updated - - -
According to the Wowhead comments the invasion quest simply does not appear if you have a level three garrison. I guess if that hasn't been fixed yet then you won't be able to complete Frostfire Ridge, which is...pretty dumb.
(You can, however, pick up the followers you would've gained from the final quest by buying them from a vendor in the town hall.)