You can pull the agent from the NS server via this URL:
http://NSName/Altiris/NS/Agent/AltirisAgentDownload.aspx
You could e-mail your users with this URL and a set of instructions.
Alternatively, you can install the agent via aexnsc.exe. You can find this on the server, and it's the same file that AltirisAgentDownload.aspx uses:
aexnsc.exe -s - a ns="yourserver.server.com" nsweb="http://yourserver.server.com/Altiris" /s