How do I apply a membership role to existing WordPress members?

We are working with an existing WordPress site, having ~5,000 members. We want to install the Membership plugin to control access to some or all content – requiring users to login. In a test environment, I see that we can select members in the membership plugin view, then choose a bulk action to add a role to all selected members. However, in the test environment, we only have a few users. We want to make sure that role can be applied to all existing subscribers. Is there a quick way to do so? Thanks!