Can individual sharing of a published article form a copyright violation?
Sending a published article to a colleague, or to someone else who has requested it, or even to a co-author, almost certainly qualifies as distribution, which is one of the six rights reserved to the copyright holder under US law. So yes, technically it would be illegal. There is a fair use "exemption" that could be relevant in some cases, but that varies depending on the specific circumstances.

Whether this also applies to a preprint or to an earlier draft depends on whether those things are included in the copyright transfer agreement.

Of course, as mentioned in the comments, it's unlikely that a journal publisher would care enough to actually make a court case out of a researcher sharing just a few copies of a paper. Honestly, it's unlikely they'll even find out.