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:
- The six friends, who had gone to school together, went to the beach.
- The six friends that had gone to school together went to the beach.
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 word "who" in the first usage above would be replaced with "which", but. Replacing the word "that" in the second usage could not accommodatewith "who" would be reasonably common usage when discussing people, but comparable replacement with "which" evenwould be less common, especially when the subordinate clause modifies the subject of a sentence. "The six machines which weren't working this morning have been repaired" would read slightly less naturally than "The six machines that weren't working this morning...", though it can accommodate "who""I have fixed the six machines which weren't working this morning" would be fine.