There are currently two ways to follow teams in the calendar section. (You can see videos of both below.) It works the same way for iOS and Android.
Following them ensures you will see their matches at the top of the calendar in the 'teams section.'
1) Press the search button at the bottom, write in the name of the team, press on it and then press the star.
If you want to add notifications, click on the bell, next to the star and then choose to use the default option or press customise to choose custom notifications.
2) Go into the calendar and then press edit in top right-hand corner.
From there you can search for the competition you want to add and then click on it. It will then be added to you followed competitions.
If you want to add notifications, you need to click on the name of the team in the team list to be taken to their match page.
Then click on the bell, next to the star. Then you can change your notification settings for that team.