Having friends is always nice and having friends in Bravely Second can certainly help out a lot! You can register other Bravely Second players as friends via the "Add Friends" option in the Save menu (when you speak to the Adventurer or outside on the world map).
Friends can be added locally or via the Internet. If you have a Bravely Second player nearby, obviously you'll want to add them locally. In which case, both players must choose the Register via Local Wireless option and one must select "Host Unit" and the other "Client Unit".
When your friends are afar, you should use the Register via Internet option while connected to the Internet. The game will search your 3DS or 2DS Friend List for friends who have played Bravely Second and automatically add them as Bravely Second friends.
If you're not aware, you can populate your 3DS or 2DS Friend List by exchanging 12-digit friend codes . To access your Friend List, select the smiley face icon along the top of your 3DS or 2DS Home menu. The Friend List will also display your personal friend code.
Benefits Of Friends
When you have friends, you can use Summon Friend to summon your friends' characters in battle. Unlike guest summons, you will gain affinity each time you use a friend's summon, boosting the strength of future summons from the same friend.
Also, you can use Abilink (found under Tactics in the party menu) to borrow any abilities your friends have learned. How this works is that you can link one of your characters with a friend; afterwards, that character can access any abilities learned by that friend.
Note : You can only borrow abilities from jobs that you've acquired.