To get a list of all users that belong to a given role, the Microsoft Azure Active Directory module has a cmdlet Get-MsolRoleMember, however to run the cmdlet you must use the RoleObjectId parameter and provide a value. The possible values for RoleObjectId can be retrieved by running the Get-MsolRole cmdlet.
To simplify this, I wrote the Get-MsolRoleMemberDetails cmdlet. As you can see from the below screenshot, the list of available roles is dynamically populated.
For this cmdlet I am using Dynamic Parameters as explained in great detail here by Ed Wilson, the Microsoft Scripting Guy.