This has been a long waited and requested feature: being able to automatically assigned licenses to user based on their group membership.
Good news, this is now available in preview 
To try the preview you will need to have an Azure EMS trial but when it will become generally available it will be included with Office 365 E3 and similar.
Also if you have an Azure Premium P1, you will be able to take advantage of the self-service portal.
I would recommend to setup dedicated groups to manage the automatic group based licensing; you can also even use dynamic groups.
Assign the license
- Logon to you Azure AD directory using the new Azure management portal (https://portal.azure.com)
- Access your directory and reach out the Licenses section
- Then click on All products
- Select the product you want to enabled for group based licensing and access the Licensed Groups section; as this the first time you are going to use this feature, you should not see any licensed groups
- Click the Assign button and follow the wizard to select the group(s) you want to use
- Then select the product(s) to enable for this group
- Then the selected group(s) will appear in the list and the state shows Change pending while setting up
- Once everything is ready, the state changes to Active
- Finally if click on the group you will be able to check who is member of this group, which services are enabled… and update if required
If already have an Azure Premium P1, you can also take advantage of the dynamic group to grant the licenses; the process is similar, except off course you have to create the dynamic group prior
NOTE if you are running an Exchange or Skype for Business Hybrid configuration, I would recommend to use dynamic group for license assignment and use the advanced rule to exclude users hosted on-premises to not enable Exchange Online or Skype for Business Online services for them
Once this done, you then will be able to see in the Licensed Users section all users enabled for the service and the assignment set to Inherited (<the name of the group used to assign the licenses>)
