There is no simple answer to your question but th e link below may help you learn how we write an invitation in formal way.
If you are going to write it in email, the format may be simpler.
An invitation's contents in order:
Title: 〜のお知らせ、〜のご案内 など
Season's greeting(formal): （時候の挨拶：じこうのあいさつ）http://www.letter110.net/zikounoaisatsu/
I would just simplify the greeting like "いつもお世話になっております（to people outside your community like customers)" or "みなさん、お疲れ様です（to people within your community)" or anything if the party is pretty casual.
Summary of the event: Simply write what the party is and what's good about it for participants.
Message: The simplest way to say "please come to the following event" I think is "下記のイベントに、ぜひ来てください。”
Due date: If you would like to set the due date of reply from participants, you can write like "ご参加いただける方は、○月○日までにご返信をお願いします。”.
Venue, Date & Time etc.: Please take a look at the link for more example.
日時 2015年X月X日（X） 午後○時○分～ ※Don't forget to write 曜日.
場所 Place name, Room name(if any), Address (People can find the place with google map!)
最寄駅 The closest station's name (if the place may be new to participants)
当日の連絡先 mobile phone number and its owner
Hope this helps!