You can get a product from ramp networks, inc. called Webramp (www.webramp.com)
It basically is a proxy server in a box allowing multiple users to access the internet at once. I have used it in one of our remote offices before, giving all users on that network access to the internet. The particular model we had allowed us to connect up to three modem lines (we only used one), but allow all x amount of users to share that connection through a proxy.
You can get different models for dsl, isdn, analog (phone), you will have to check with the provider and / or webramp to see what is right for you.