Invitation letter is just like any other letter. The only difference is in the text of the letter. An invitation letter is written to invite some one on some important event. You can write it as,
Name of Person,
Designation,
Address
Subject: Invitation Letter for ...
Dear Sir,
This letter is to invite you to attend the Annual Seminar of our College on (date). Your participation would be highly appreciated on the occasion. I hope to seeing you on (date).
Thanking you in anticipation.
Your Name,
Designation,
Address.
Name of Person,
Designation,
Address
Subject: Invitation Letter for ...
Dear Sir,
This letter is to invite you to attend the Annual Seminar of our College on (date). Your participation would be highly appreciated on the occasion. I hope to seeing you on (date).
Thanking you in anticipation.
Your Name,
Designation,
Address.