The friends feature in Soccer Stars only works for Facebook-connected accounts.
Go to the main page of Soccer Stars. Press the "Friends" icon in the bottom left of the page.
Here, you will find a list of all your friends on Facebook already playing Soccer Stars.
You are able to challenge them directly from this list, just by pressing the challenge button and selecting the tier.
The edit button allows you to remove some friends from that list or invite others to install and play Soccer Stars with you.
You are also able to add new friends that you don't have on Facebook. Press "Add new friends" in the bottom of that page and search for the name of the person, or better yet, enter the Unique ID if you have it.