Since I’m using the app right now, I’ll have to recite this from memory. But at the bottom of the screen you should see a button second from the left three lines.
Click on that and search for a group in the top portion. Click on the group once found , and there there is a subscribe button. As of yesterday cliking it appears to do nothing. In fact it does work. Exit the list and go back in, you will be subscribed.
O.o