The plugin mentioned by swissspidy does exactly what you want, but it has not been updated for a year, and is not tested with the latest WordPress version. This is why I will also add 2 other solutions here.
The Pending Users module of my premium plugin Beyond Multisite is made to do this. Link to the plugin page: https://codecanyon.net/item/beyond-multisite-utilities-for-wordpress-network-admins/19633352
On the following screenshot you can see the page that the plugin creates and the actions it offers (last column) for each pending user. The plugin has many more features unrelated to what you are asking too.
To activate the signup, simply visit the Pending Users page in the Network Admin Dashboard under Users, and click the Activate link in the last column for the row you want.
This solution is a bit more technical, and is good if you rarely need to do this. Also it is free, and there will be no plugin required.
You need to go to the phpMyAdmin panel (usually located in the cPanel of your hosting) and open the database for your multisite (the database name is visible in your wp-config.php file).
Now open the table wp_signups (your prefix might be different than wp_) and there find the signup you want to activate. In the column activation_key you will see a string. Add this string in the place of XXXXXXXXXXXX in this URL (where example.com is your main site URL) and visit the URL to activate the signup:
Basically this is the link that the user receives via email in order to activate the signup.