The way you've set it up, probably won't work very stable.
You need to point other computers to your iMac in their DNS settings to try and find your site which is a lot of work and needs to be monitored constantly.
Best way to go for now, set your iMac to a fixed IP address and then add a bookmark in their browser of choice for all other computers on the network with the "intranet" assigned to it; for instance http://10.0.0.10/intranet/ bookmarked as intranet. That will work as long as your iMac is available on the network.
That should work ok for now, but consider investing in a more robust setup where a dedicated server with perhaps MountainLionServer makes your life a lot easier.
With OS X systems and iOS devices and other systems with support for Bonjour (mDNS), at least until you have DNS services enabled and configured on your network...
With Bonjour, your web server address is your system name with the Bonjour top-level domain ".local" appended. If you've named your system "foo" for instance, then type "foo.local" into the address bar of your web browser.
Longer-term, register a domain name, acquire OS X Server package for OS X (as Jepping suggests) or acquire some other opersting system with a DNS server, and get DNS services set up on your internal network.
If you want to use the static-addressing mechanism mentioned by Jepping, you'll want to make sure your iMac is using an IP address that is both within the IP subnet you're using and also outside of the address pool issued by your DHCP server, or you'll want to update your DHCP server to always issue the same IP address to your iMac MAC address.
This question is "urgent"? Um, seriously?
If you're willing to hand-edit stuff and do some research into the details of the set-up, then IIRC Snow Leopard client did have the various pieces of DNS services latent, but it's entirely manually managed, and involves editing various configuration files. Using OS X Server is and would be far easier.