For those who haven’t yet used it, Slack is a powerful communication platform, based around a chatroom-style way of interacting. I won’t go into details here, except to say that it also has an API that provides various methods to interact with the system, many of which require you to supply a user ID to act on, which is not visible in any of the admin UI sections.
Therefore the easiest way to get IDs for your users is to call the
users.list method, which you can run from their test harness at https://api.slack.com/methods/users.list/test. This will give you a full list of users along with their IDs ready for you to use.