When you create a user using the UserManager in the arcgis for python API it currently does not let you modify in anyway the boiler plate email sent to that user, however this functionality is there if you look at the code at __init__.py in the GIS module, you just need to change and add a few lines. This should be added officially to the next release of the API. It's a useful functionality, for example in my org we have an application for our external customers and there is some extra information we want to convey to them when we add them to our organization and we'd rather not have to send two emails, one from arcgis and one from us as this can confuse some users or they'll miss one of the emails
