The Provisioning API that you linked to is deprecated and should not be used for new development.
The new Admin SDK Directory API should allow you to perform these operations.
To get a list of all groups where the primary address is one of your subdomains, use the groups.list API call and specify the domain parameter with your subdomain.
To get a list of mobile devices associated with users in the subdomain, use the mobiledevices.list API call and specify customerId=my_customer as well as query=email:subdomain.com parameter.