To write an invitation letter for a UK visitor visa application, you need to include the following information:

• Your name, address, contact, job title, and financial situation
• The name and date of birth of the person you are going to invite, and the duration of their stay
• Information about if you are going to provide the accommodation for the visitors
• Information about if you are going to sponsor the whole trip

For more update and accurate information, you can contact UK Border Agency or visit their website.

The following format of an invitation letter may help you to start:

Date of the letter
Your name
Address and contact number

To Whom It May Concern:
Application for visit visa: name of the people you invite
I am writing to confirm I wish to invite (the name of the applicant) to visit me in the UK for (the length of the duration) for (the reason of visit such as holiday, family visit) . I hope the applicant(s) will be able to come on (the date of UK entry).
Provide the following information: your UK visa and your nationality, your job and accommodation details.
I am able to accommodate the applicant(s) for the duration of their stay in the UK.
I have enclosed the following documents:

• Copy of passport
• Bank statement
  
• Tenancy agreement

Should you require any additional information please do not hesitate to contact me.
Best regards,
Your name
Signature

If you are student studying in the UK, the requirement of document will be slightly different. You can check the sample here.