Did you make sure they joined the same world as you? If you aren't on the same server, you can't be in a FS together. Once you confirm that, are you playing on mobile, or in the browser? I don't know if there is a way to search for people in the mobile app. In the browser, Herbertus's info will help you find them:
Click here:
Then type their name in the search bar:
and hit the magnifying glass.
It appears you are playing in Felyndral. If they are not, they need to (at the startup screen), pick Felyndral to create a new city so you can find each other