To import users from Slack, there are very simple steps you have to follow:
Note: if you don't have the button [Import employees from Slack], this means you have toIntegrate with Slack first.
Note: In order to be able to import correctly your employees from Slack, please make sure they have setup both First Name and Last Name in your Slack account. Slack gives some tips about how to configure this https://get.slack.help/hc/en-us/articles/204092246-Edit-your-profile"