It's worth noting that the term "who" with a preceding comma often has a different meaning from "that", and the latter term would not substitute.  Consider:
<ol>
<li>The six friends<i>, who had gone to school together, </i>went to the beach.
<li>The six friends <i>that had gone to school together</i> went to the beach.
</ol>
In the first sentence, it is assumed that the reader would know, even before reading the italicized portion of the text, what the six friends the author was referring to, but not that they had gone to school together.  In the second sentence, it is assumed that the reader would know that six friends had gone to school together, but not that the author was writing about those people in particular.

When discussing people, the second formulation could use "who", though such usage would make it harder to distinguish from the first.  When discussing inanimate objects, the "who" in the first usage above would be replaced with "which", but the second usage could not accommodate "which" even though it can accommodate "who".