Dear Mr Fred
Hello! We would like to invite you to a seminar in our University! There will be many interesting and educational. So you can say your speech and share the experience with others. Or to learn something new for yourself! It will be very fun!
(эти две строки просто для полноценности, они там есть:)
I am looking forward to bearing from you.
Sincerely yours,
(А на следующей строке пишешь своё имя на английском:)