Project Resource Management efficiency

Summary: Hello, we have a body of projects for customer support. There are usually between 200 and 300 open with a constant stream of new projects added and old closed. We have a pool of about 250 resources. Any one of these resources may be called on to work any of the open projects. We are finding it very time consuming to add each resource to each project. Is there a way to either create group and then add the group to the project so we are only managing resources in