To add users to Sales Assistant, go here. You will see a table containing all the users in your Salesforce instance that looks like this:

Check off the users that you want to add to Sales Assistant, and click the Save button at the bottom:
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To remove a user from Sales Assistant uncheck their row in the table and click the Save button.

The users that are added to Sales Assistant will be visible in the Sales Assistant Leaderboard and will be notified about their open nudges. You can change how they are notified in the Notifications tab and which nudges are computed for them in the Settings tab.