I'm using an Internet Provider that requires a username and password to log in; the prompt shows up as soon as you open a web browser. Because of this, I can only have one device online at a time. How can I get my Time Capsule to essentially log in as the user and allow other devices to connect through it. My Time Capsule has worked with other ISPs that don't require the user log in, but I recently moved. I may be wrong about how the ISP is allowing access, I wasn't sure how to describe the problem. Thanks.
Turn on NAT ("Share a single IP address"). That way only your Time Capsule's MAC address will be seen by the ISP. Then whenever you need to authenticate via the web page, it doesn't matter which machine on your home LAN/WLAN authenticates, that authentication transaction will appear to come from the MAC address of your Time Capsule.
If turning on NAT doesn't help, then your ISP is doing something other than just looking at the MAC address.