Every day go through all open projects and any service provider who have expired that would be a good match for the project should be contacted to make sure they didn’t miss the project. For email, there is a Yesware template for support in “Support > Project Expired” that can be used.
If you need more service providers, beyond recruiting make sure you go to the Coverage Map tool available to MB admins. Center the map on the project location and look through the service types to find other related services that may have service providers you can recruit. Take the final list of service providers, look up their email address, and invite them into the project.