Fighting over Ethernet ports can get ugly. The option of switching back and forth between computers never works out very well. And somehow no one ever learns: Please install more jacks!
You could always just carry around a portable router, but then you'd need an extra place to plug that in too. However, with a Mac, you can turn one connection into many. Sharing allows you to funnel the power of one cable into another, or turn your computer into a wireless broadcaster.
To set up an extra computer, go to System Preferences in the Apple drop-down menu in the upper left-hand corner of your screen. Under Internet and Network, select Sharing. Make sure Web Sharing is enabled by checking the box in the menu on the left. You can't enable Internet Sharing without doing this first.
Now highlight Internet Sharing in the menu. Select the source (Ethernet), and the connection point (Airport, or if you have an extra Ethernet cable, Ethernet). Then, go back to the left menu and click on the check box. A warning will appear, telling you that doing this while connected to a network (like you would find in an office or school) could cause problems.
As long as you're not on a network, click Start. This sets up a wireless (or wired) connection between your Mac and another computer. The other computer doesn't have to be a Mac either. You can even do it with a PC!
In order to keep this connection secure, set it up as a network first. Instead of starting at System Preferences, click on the Airport icon in the menu bar. Select the option to "Create a Network". Name your network, leave the channel on "Automatic", and check the box next to "Require a Password". You'll have to type in a password twice. Share this password with the people you will share the internet connection with.
Once you've got the network set up, go ahead and enable Internet Sharing as described above. Only people with the password will be able to connect, which is better security and will keep the network from getting too slow.
Now that you're sharing your Internet connection, you should never have to worry about the lack of jacks again.