Yesterday I joined a Spartan company and they have already completed the 31 commendations required to get the Achilles pack but I didn’t get it. When I joined it said that I got the pack but when I launched the game I didn’t have any unopened packs. I went to my customization to see if I’ve gotten the armor automatically, but it wasn’t there either!
If you join after a company has already achieved the Achilles armor then I’m afraid you won’t be able to get it, you need to have joined the company before they achieved the armor
As stckrboy said, you have to be in the company at the moment when it unlocks the commendation mastery to get the Achilles armor set. Here is a quote from the Infinity’s Armory update description that details this:
> Note that you have to be a member of the Company at the moment when the award is given to earn the goodies. So if you want it, you’ll need to join a Company that does not yet have the armor or helmet. No bandwagoning here. Stick with your crew.
Full article here: https://www.halowaypoint.com/en-us/news/infinitys-armory-preview-2