Start by formally addressing the recipient. Include the business address and designation in the lines under the name.
In the opening paragraph, state clearly the purpose for the proposal or request. Depending on the nature of the invitation, include details that are relevant and explanatory regarding your request.
In the second paragraph, give details of the outcomes you expect as well as what has led you to decide to make this request or proposal. Be clear, honest and direct.
Maintain a respectful and polite tone throughout. Show sincerity and eagerness in extending the invitation. Add a personal touch at the end by saying how much it would mean to you if it is accepted.
End by signing your name and giving your address and number where you can be reached.