You'll need to send out the meeting invite before you can assign roles. Once you've done that, go to Calendar, click on the meeting you just created, and select Meeting options. This will bring you to a web page, where you'll see a few choices under Who can present?
Everyone: Anyone who has access to the meeting link will join the meeting as a presenter.
People in my organization: Only people in your org will be presenters. External participants will join as attendees.
Specific people: Only people you choose from the list of invitees will be presenters. Everyone else will join as attendees.
Only me: Only the organizer will be a presenter. All other participants will join as attendees.
During the meeting: Click Show participants in the meeting controls to see a list of all the people in the meeting -> The name of the person whose role you want to change and click More options -> select Make a presenter or Make an attendee.