You should read the JP translation thread, where it talks about "Free Company." Which sounds like people will be able to obtain items from any company through it.

There isn't much difference on which country you enlist in, just varies in how the event is viewed (cut scene)
- To avoid one country getting too much enlistment, making it so depending on number of enlistee the company bonus changing.
- It doesn't mean you can't switch (grand company) later on, later we are looking to adding "Free Company" where you can join optionally