Find the three dots on the upper right corner of your Slack app.
From the options showed when you click on that, select 'Wokspace Directory'.

Next, click on the gear icon to open your directory.

Locate your member using their email address/username on your Workspace Directory. By default, they won't automatically appear and you get this message:

Just click on 'Show all members' then click on the three dots for the 'Activate account' button to appear.
 

That should re-enable your member's Slack access. You can then send them a Slack email invite using Slack's group finder here: https://slack.com/signin/find

Here's also a guide from Slack utilizing the workspace name in the top left:
https://get.slack.help/hc/en-us/articles/204475027-Deactivate-a-member-s-account

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