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