To avoid namespace collisions on azurewebsites.net I prefix my websites with a string. Example foo-app1.azurewebsites.net and foo-app2.azurewebsites.net, etc so in this case they are grouped under the "foo-" prefix.
However a subdomain would be a nice way of grouping websites.
Is it possible to group websites as app1.foo.azurewebsites.net and app2.foo.azurewebsites.net?
Is it possible to group websites as app1.foo.azurewebsites.net and app2.foo.azurewebsites.net?
For azure app service, app1.foo.azurewebsites.net and app2.foo.azurewebsites.net are two independent web apps. In order to group your websites, I assumed that you could configure the custom domain name for your Azure websites.
Assuming that your custom domain name is contoso.com. You could map your subdomains against the same azure web app as follow:
app1.contoso.com -> {app_name}.azurewebsites.net
app2.contoso.com -> {app_name}.azurewebsites.net
Also, you could map your subdomains to your different azure web apps as follows:
app1.contoso.com -> {app_name_01}.azurewebsites.net
app2.contoso.com -> {app_name_02}.azurewebsites.net
More details about how to map your custom DNS name to Azure Web Apps, you could refer to here.