I am no expert on this matter, but my application was successful, so I will share what I asked my sponsor to include in his invitation letter in the hopes that others will find similar success. I will update as/when I find out more info via research too.
Here are the things that were included in my invitation letter:
This is all I included and my application was successful. However I may have been lucky and/or had a strong application apart from the letter, which may have helped, and therefore I will be doing some research online and updating my answer with any additional information that others have used in their Sponsorship letters.
Here are a few other things that might be good to include:
Credit:stackoverflow.com‘