A good captain never loses contact with their trusty mate(s).
Unlike Public Chat or Guild Chat, Crew chat is only received by those pirates in your establishd Crew.
In the Chat Box at the lower-left, select Crew. Anything now entered in your Chat window will appear in GREEN and only fellow crew members can read it.
Crew members can chat with each other, no matter the members are. You do not need to be close to another crew member for this to work. However, you will only remain in the same crew as long as you are on the same server.