mutualGroupMembers = async (user, group) => {
  let myFollowings = await db.query(
      'SELECT follow_system.follow_to AS user, follow_system.follow_to_username AS username FROM follow_system WHERE follow_system.follow_by=?',
      [user]
    ),
    grpMembers = await db.query(
      'SELECT group_members.member AS user, users.username AS username FROM group_members, users WHERE group_id = ? AND group_members.member = users.id ORDER BY group_members.joined_group DESC',
      [group]
    ),
    mutuals = intersectionBy(myFollowings, grpMembers, 'user')

  return mutuals
}
Returns mutual users of group members and user
@param {Number} user UserID
@param {Number} group GroupID