You may either manually enter an invite link through the Join Server menu or hit immediately on an invite link from a friend to automatically join the server.
Here you can start it by pressing the green "+" sign button in the left sidebar to manually enter an invite link that you've received!
The option to create or join a server will appear after that. Choose "Join a server" and then manually type in the invite link you received! To confirm, press "Join Server" after you've filled out the relevant invite URL.
Before you formally join, the app will double-check that you're on the right server.