You must have an internet connection on your SI Server machine while registering.
You only have to register SI from one SI Client machine on your network.
You should have received your Product Key for SI via email.
The first time you launch a SI Client on your network and log in (same domain/workgroup where your SI Server is installed), you will be prompted to enter your SI Product Key or to register for a free 30-day Trial.
Registration is required after the 30 day trial period has expired. To register, click the [Register Now] button on the Home Page of a SI Client:
If you don't see the [Register Now] button on your Home Page, you can always go to Start->Help->Register:
You will then be prompted to enter your Product Key: