This covers how to define additional users as local admin on Azure AD joined devices.
CREATING AN AZURE AD GROUP FOR YOUR ADMIN USERS
Within Azure Active Directory, you need to create a group that will be used to define which users should be added to the local administrators groups for AzureAD joined devices, important if your environment is built upon Intune/Endpoint Manager.
The group needs to have the 'Azure AD roles can be assigned to the group' set as YES. This is so AzureAD can assign the local device administrator role to the users in that group.
SETTING THE GROUP AS LOCAL ADMINISTRATORS ON AZUREAD DEVICES
Within the Azure Portal under Devices, select the 'Device Settings' section then at the bottom is 'Additional local administrators on all Azure AD joined devices' where you define additional local administrators.
The link should open the screen shown below. You can use the 'Add Assignment' to add your AzureAD group to be used as local administrators on all Azure AD joined devices.
Once you have set your group with the assignment, you should be able to log into an Azure AD joined device and automatically be added as a local administrator.