I am looking for a way to add a list of computers from a text file to a SCCM collection. Is there a way to do this? Manually adding each computer to a collection using the direct membership wizard takes too long for many PCs.

Sure, there are several I've seen, or you can roll your own based on the SDK. But my argument would be "why"? Direct Membership collections can have their drawbacks. If someone hands me a list of computers that they want added 1 by 1, I usually try to push back and determine what they really want: The computers in OU XYZ to be in that collection? The computers with Widgets version 3.0 in that collection so they can be upgraded to the next version? A query based collection is a better answer usually.Standardize. Simplify. Automate.