Note - This is an EHQ Enterprise Edition feature.
With the enterprise edition of hubs, there will be an additional filter available to group participants by Hubs. The article on Creating a group of participants using filters discusses all the different filters that can be used to group participants.
In the Participant page, you will now find an option as highlighted below called Hubs. You can select your Hub or search for it. This will filter the participants and displaying only the participants of the projects aggregated by the selected hub.
You can then choose to create a group or review the list of participants in the hub.