On a Full Access plan with a 100 or 500 participant limit you have the option to 'boost' your plan. Boosting allows you to increase your meeting participant limit to 1000 for a period of 7 days from the time that you purchase the boost. A boost can also be extended for a further 7 days at a time.
You will see a 'Boost' option in the Account tab of your dashboard. Boosts can only be purchased via credit card.