This seems really useful, thanks for making the mockup! Some thoughts:

I'm thinking that we need to tell people what to expect from the application process before they are confronted with the 'this is a closed group' dialog box. That seems very abrupt to me. Similarly, I think that people should find out about requirements before they get denied, as finding out along with a 'deny' seems likely to cause people to feel rejected and not bother trying to fullfill them.

Similarly, if it's a closed group, there should be some sort of notice when you try to join, rather than upon being denied. Then someone can contact a group member, rather than go through the application process only to be denied.

I also don't think 'join hub' should show up until one has subscribed, but maybe that's me.

I believe that some hubs want to have a conversation with someone before accepting or denying them; as such, I would expect the ability to chat or send a PM to someone, rather than just 'accept' or 'deny' as options. I realize that there is also the option to visit their profile, and maybe that's the intent. If so, though, I'd think that the profile visiting affordance needs to be more prominent.

Hopefully these are helpful!

First, the main page view, before subscription or joining:

First, the main page view, before subscription or joining:

When you click on 'subscribe', this is the next page shown:

When you click on 'subscribe', this is the next page shown:

If you select 'join', a dialog pops up, like so:

If you select 'join', a dialog pops up, like so:

Upon send request, admins have notifications sent to their stream as well as showing up on the hub page.

First, the hub page with a single request:

Upon send request, admins have notifications sent to their stream as well as showing up on the hub page.

First, the hub page with a single request:

The hub page with multiple requests:

In both cases, the name is a clickable link to the person's profile page.

The hub page with multiple requests:

In both cases, the name is a clickable link to the person's profile page.

Now, a request as a notification on someone's stream. It may not be immediately obvious, but the blue bordered notifications are those requiring actions. The relevant notification is the fourth one down.

Now, a request as a notification on someone's stream. It may not be immediately obvious, but the blue bordered notifications are those requiring actions. The relevant notification is the fourth one down.

And, clicking on the notification, which brings a similar interface as the single request on a hub page:

And, clicking on the notification, which brings a similar interface as the single request on a hub page:

If you have multiple requests and click on the set (including 'manage...'), you get a dialog with multiple requests to deny or approve:

If you have multiple requests and click on the set (including 'manage...'), you get a dialog with multiple requests to deny or approve:

Next is a dialog wherein some of the people have been denied or approved, and one remains to handle:

Next is a dialog wherein some of the people have been denied or approved, and one remains to handle:

Finally, the 'deny' auto-populated options. When you send or cancel, you are back where you started:

Finally, the 'deny' auto-populated options. When you send or cancel, you are back where you started: