Friends are great! Friends give you honor when they use your hero as a reinforcement. Honor is used to summon heroes.
To add players to your friend list, go to a dungeon, select a reinforcement, complete the dungeon, and you will receive a dialog asking you to send a friend invite. When they accept your invite, they will appear on your friend list.
Your friend list can be found under the Guild tab. You will also see friend invitations from other players on the friend list.
Your guild mates are not automatically your friends, but if you happen to see a guild mate on your reinforcement list you can invite them.