Run Alpha again! Consider inviting Alpha guests to help on your next Alpha as a team member!
Remember that your best tool after Alpha is the Church. Encourage the guests to become part of your church community whether that is by attending another course or a small group. Make sure to host them well if you invite them to church on a Sunday morning.
Here is a selection of courses we recommend once you have completed Alpha - https://shop.alpha.org/t/AfterAlpha