Your friends in Senbura are listed as your followers.
You can opt to follow people; if they do not follow you in return, you can still see them on your followed list. Other people can also opt to follow you - if you use them after a battle, you have the choice to choose if you wish to mutually follow.
Mutual following will earn both parties gacha point rewards.
Followers can be added either by ID-input (if you have the ID) or adding from a random list of support accounts shown to you before battle.
Only the follower's leader character will be part of your team during a battle.