Hi Drew,
Thanks for posting about this. As Josh said, you can create additional inputs by dragging another connection to the input side of the Cat node.
In an effort to make this functionality more easily discoverable, we will be adding some placeholder "halos" to provide a visual cue to the user like in the following screenshot:

We are planning to roll this out to users in an update in early 2017.
Thanks again for your question, and please don't hesitate to share any others you might have, no matter how basic, as this feedback is a big help as we prioritize future usability improvements.