If Safeguard is installed in the system, there is also the possibility to use the SAFECOM ADD USER command. It allows more direct control of some of the attributes of the userid being added than ADDUSER does.
You can access all of the NonStop manuals by going to:
http://www.hp.com/go/nonstop-docs
On that page, scroll down to the heading "NonStop Technical Library". Under that heading, click the link that includes your OS release in it (H-Series, J-Series, or G-Series). The page that takes you to is a list of all the manual titles in alphabetical order. Each title is a link to the .pdf file of the manual.
Sometimes the product name is not the first word of the title, so if you do not find the manual you want where you expect it to be, use the web browser's text search to search for the product name on that list of titles web page. Often there are several manuals for a product, for different audiences. Sometimes there are several versions of the same manual listed. So look over all of the titles for the product to be sure you choose the one best suited for your needs.
For your current question, the "Safeguard Reference Manual" probably would be the best to use to learn about the SAFECOM ADD USER command. The "TACL Reference Manual" that dimandja referred you to probably would be the best to use to learn about the TACL ADDUSER command.
Sometimes the title listed in the list of titles web page is not exactly the same as the formal title of the manual on its title page. For example, the "TACL Reference Manual" actually is titled "HP NonStop TACL Reference Manual". Don't get confused by the difference in the titles.