In the past, it was considered that a certain kind of seriousness is needed for the business development but then another school of thought decided to add some excitement in the dull and boring routine of office life. They came up with idea of business parties during the course of a year.
Now many companies are following this technique and a lots of parties are organised in or out of the offices. In case of such plans, an official announcement or invitation is always required to inform all the employees and associates about the occasion. The person who is responsible to spread the word should ensure that everyone on the guest list gets complete information of the location, date and time.
In the opening paragraph, you should invite the addressee by providing the complete information of venue, date and time. You should give your receiver clear idea about the nature of this gathering.
The next paragraph should comprise instructions about the dress code etc. You should be brief and clear to avoid the margin of confusion.
In the end, add some courteous sentences to make the other party realise that this invitation letter is not a regular/formal notice. Remind them to inform you about their status and then close your letter with proper salutation.