Once the license is set up, anyone on the computer can use it - you can use a non-admin account to run Houdini normally.
I expect that you'll need to use the administrator account again to refresh the license when it expires, every 30 days or whatever.
If you didn't want to invoke Houdini itself from the administrator account, you should instead be able to just run the license administrator, which is here:
/Applications/Houdini.../Administrative Tools/License Administrator.app