It's a function that will become available once both commanders have unlocked Command Center level 30.

You may add up to 50 friends, and no further friend requests can be sent once this limit is reached.
After a friend request is accepted, commanders can communicate through private chat and exchange gifts.

Note:
-The gift-sending feature is unlocked once the Command Center reaches level 40.
-Private chats have a message limit, and older messages will be automatically deleted once that limit is exceeded.
-Users can mark certain friends as "Starred Friends," and these commanders will appear at the top of the friends list for easier access.
-Commanders listed in your blocklist cannot be added as friends unless they are first removed from the blocklist.