Dear (имя кого приглашаешь),
Two of my friends from Spanish arrived in the morning today. We are going for a picnic on a park July 10. We shall go there by a taxi.
We will start our journey at 7 am and return till 9 pm. Park has many beautiful places and is of historical significance. I insist upon you to try your best to accompany us. It shall be a great pleasure to have you in our company.
My regards to your parents.
Yours sincerely,
(Своё имя)