Speakers should be researched and chosen. After being chosen, they should be sent the information related to the event. The requirements of the speaker should be taken before hand, for example, audio-visual requirements, accommodations etc.
The best possible way to get people to attend would be to get renowned people of the related field to attend the conference. This stirs the interest of the targeted audience and gets the right people to attend.
As mentioned, getting renowned names often stirs interests, so get names of the field to the event, but do not completely go off topic from the entire event’s focus. Also make sure that you’ve taken the requirements of the speaker before hand and arranged for everything that is required