First, go to your Org Members page: Click ORGANIZATION in the main nav at the top of the screen.
Then, click the "Show Archived" checkbox next to where it says Groups.
Once you scroll or further filter the list of people to find the person you want to unarchive (their name will be a lighter color of gray), click their row to open the details panel.
In the details panel, click the three-arrow recycle icon above their name to unarchive them.
Their account will be restored, but no other actions will be taken (no email notification).