This question may need to be asked again to get an exact answer. What kind of invitation do you mean? Normally you don't need any kind of government document to write an invitation.

If you are thinking about inviting friends or members of your family to live or stay with you, in the UK for example, then you may need a government document, such as a visa, depending on where you or they are from. You can find the information you need for this at the UK Border Agency. You can also contact the agency directly if you don't find the information you need on the website.

Otherwise I'm afraid I'm not sure what you mean. If you would like to ask your question again, and give some more information about what kind of invitation you are talking about, I'm sure someone will be able to help you.