You can select any empty town slot on your land and then click open the Invitation Pop-up: 

Select an empty town slotInvitation pop-up

Click Invite Player button and provide the player ID (for existing players) or provide an email address to invite someone new to join the game. Please note that you need to copy the generated invitation text and send it to your friends via email. Medieval Empires cannot send any email without user consent. 
 Provide the player ID of invitee, or email address for someone new to the game

Generate invitation message for Email address:
 Generate invitation email text  

This is how invitation texts look like:

Hi my dear friend,

Big news! I'm now a landowner in Medieval Empires and I want you to join me in the game! 🏰 Let's build our empires, conquer territories, and kick some virtual butt together! 🌍💪 Are you ready for an epic gaming adventure? Let's do this! 🔥

#MedievalEmpires

Follow this link to sign up on Medieval Empires

https://account.apps.medievalempires.com/signup