I still feel like creating a group owners membership management page will be much better for us in the long run instead of the “request membership via a PM” approach. We can just send a notifications to all group owners notifying them of a pending requests queue and use a button to grant the user membership.
We used a PM in the 1.7 release because it was the easiest thing without having to spend too much time on it. Having special triggers in a text can quickly become a nightmare to handle based on my experience building discobot.