Information from CIC:
http://www.cic.gc.ca/english/visit/letter.asp
Sometimes a person applying for a Temporary Resident Visa to visit Canada is asked to provide a letter of invitation from someone in Canada. The following information will help you prepare such a letter to send to a relative or friend abroad.
A letter of invitation does not guarantee that a visa will be issued. Visa officers assess the applicant to determine whether they meet the requirements of the Immigration and Refugee Protection Act.
Note
The following information is only a guide. You must write the letter of invitation yourself. Some visa offices may require that your letter be notarized by a Notary Public. Check with the visa office that is processing the application to find out if you need to do this.
By writing a letter of invitation, you are not legally responsible for the visitor once they get to Canada, but you should provide the letter in good faith. You must give truthful information and intend to keep the promises you made in the letter.
Send your letter (notarized if necessary) to the person you are inviting to Canada. They must then submit this letter to the Canadian Embassy or Consulate outside of Canada when they apply for their Temporary Resident Visa.
Your letter must include the following information about the person being invited:
* Complete name.
* Date of birth.
* The person's address and telephone number.
* Your relationship to the person being invited.
* The purpose of the trip.
* How long the person you are inviting intends to stay in Canada.
* Details on accommodation and living expenses.
* The date the person you are inviting intends to leave Canada.
Your letter must also include the following information about yourself:
* Complete name.
* Date of birth.
* Address and telephone number in Canada.
* Occupation.
* Whether you are a Canadian citizen or Permanent Resident
* A photocopy of a document proving your status in Canada. For instance, a Canadian birth certificate if you were born in Canada or a Canadian citizenship card if you are a naturalized citizen. If you are Permanent Resident, you must send proof of your permanent resident status (a copy of your PR card or your IMM 1000 proof of landing).