Нужно составить письмо.

You have received an email from your English-speaking pen friend Robert.
...i hope you had a safe journey home. It was great spending time together. I hope you will come and visit again soon...

Write him an email to thank him and his family for their hospitality, then invite him to stay with you.

Thank you for your letter. It was very nice to meet you and meet your familyI hope you too will come to visit. It will be great, I'll look forward to meeting you and your friends. However, in my city there are a lot of wonderful sights, so it will be impossible to see them in a short time.