I made a workaround for this. I created a slack list of Names and People, then in the automation use slacks step "Select a list item" to select the correct user from the list.

The list item can then be used as a variable, by which you can access the 'people' part which can be mentioned, added to groups all that sort of thing. It's a bit of a pain cause you have to manage a separate list of names/user pairs but it makes it work as a user in slack which is nice.