There is currently no straight-forward way to do this via SSH Professional. However, there are a couple options depending on your configuration. We are currently working on building out our API to assist with such tasks, but this is still a work in progress.
Depending on how hosts are deployed, could use Ansible (or a similar tool) to read in the /etc/step-ssh-acl.json file on every host and collect that data for a total list.
Alternatively, you can go from the IdP side of things. For example, if you are using OKTA for user management, you can use the OKTA API to query users from each group assigned to SSH Professional.