In slack, is there a way for me to create a custom slack '@' command? E.g. @here or @channel notifies everyone in the channel. Can I create a custom group, such as @coolPeople that would, say, only notify John, Jack, Jill, and Jane, but not Adam and Amy? Thanks.
Yes. It is called user groups and works just as you describe.
You create a user group, e.g. named "coolpeople" and assign people to it. You can then use @coolpeople to notify them in a post. This also works form an app as described here.
One caveat: This is a premium feature and therefore not available to Slack workspaces on the free tier.