Adds a user to the guild, provided you have a valid oauth2 access token for the user with the guilds.join scope. Returns a 201 Created with the guild member as the body, or 204 No Content if the user is already a member of the guild. Fires a Guild Member Add Gateway event. For guilds with Membership Screening enabled, this endpoint will default to adding new members as pending in the guild member object. Members that are pending will have to complete membership screening before they become full members that can talk.