Click on the Task to access the Task Properties on your mobile app.
Then you will find the Followers option in the Properties section of the Task Properties.
Click on the (+) icon next to the Followers option to add followers to the task.
Select the member from the drop down menu. You can also type the name of the member in the Search Members field.
You can also add multiple members as followers to the same tasks this way.
Note: Followers will also become Project Members.