The "Join" button is not yet launched on iOS. Viewers can, however, become members on desktop or the YouTube app and still get the perks (badges, members-only community posts, etc.) on iOS. Hope this clarifies.
We are constantly trying to make this available across devices. You can share your thoughts about this feature via 'Send Feedback'. Here's the link: https://goo.gl/WdI0BK . While every individual report may not get a direct response, we do review everything.