You need friend points which are received by sending/accepting requests from other players after joining their guild or by gaining experience from quests, leveling up characters, etc. Sending a request costs 1 friend point while accepting a request costs 3 friend points. You can send up to 5 requests per day and can hold a maximum of 50 pending friend requests.
Co-op mode allows you to fight bosses with another player using two teams where each team has 2 heroes who can synergize with each other to perform certain actions. Heroes on the same team must be relatively close in terms of character level, weapon rarity, and all skills in order to work together (i.e. same type of attack).
Each kill in co-op mode rewards the player with a bag which contains jewels for leveling up. The player with the highest score (i.e. damage dealt) on their team wins the fight and scores are shared amongst all members of the victorious team.