Formal invite suitable for associates and respected community members:

Dear [name],

I'd like to officially invite you to attend Software Freedom Day, Saturday 18 September at the State Library of Victoria. As [phrase about what they do, personalised to show you know what they're involved in], I thought [you/you and your team/your department/your members] might be interested to attend the event.   

Software Freedom Day is an event held all over the world to celebrate and raise awareness of free software and open source software. We've been working hard to bring you an exciting range speeches, workshops and activities. One of the highlights this year will be a [speech/workshop] by [speaker name] on [speech topic].

We'd be very pleased to see you there!

For more information and a full list of activities, see our website:
http://www.sfd.org.au/melbourne/

Regards,
[your name]