From the way you phrased your question, I understand it to be you wanting to join an existing wireless network instead of creating one and having everyone connect to it. If that is indeed a case, you'll need a device that allows you to participate in a wireless network, the most typical being a Wireless card (e.g. Airport). Since that isn't one of your options, you could consider getting a USB-to-WiFi device, but there have been very few success stories about this. You might bump into
D-Link's DWL-122, a USB device that acts like a wireless LAN card, but Mac performance and reliability is shoddy at best. If you're able to get one with a money-back guarantee, it may be worth a shot.
Another solution would be
Belkin's 802.11b Wireless USB Network Adapter (F5D6050). It costs about $50 and Mac drivers are available. I found it from
this thread.
Getting a WAP will only work if you can configure your WAP to join the existing network. That means configuring it as a bridge to basically extend the coverage to where your Mac is. However, bridging is not a universal standard, so one vendor's implementation can vary greatly with that of another.