Depending on your audience you will have a standard way of contacting them. Usually someone in the organisation will have a central email list, but it could also be that individuals manage smaller groups and are fairly autonomous when organising events. This information should all be gathered together to plan you event to be as inclusive as possible.
Once you have all you audience in a list you can then send out you notice of intent. If you have not used this list for mailshots previously then you might want to use some List Management Software to take care of it. It is important to respect any local laws with regard to data protection.
One approach we have taken is to send out a general mailing asking everyone if they intend to attend the event. This can generate some very powerful data and enable you to be much more efficient in the future.
In one example we sent out a mailshot to all members of a society. The responses were in the form of a poll and there was an option for people to unsubscribe if they wanted to. We could also see if any of the email addresses bounced back and also see who did not respond. A few days later we resend the mailshot to those who had not opened the initial email being very careful not to bombard anyone with multiple invites.
We then tagged those users with polling data as potential attendees so we could update them on the details of the event later. The rest of the list could now be ignored until after the event when it could be further updated.