Are you sure you want to activate them all? If they are inactive, there is usually a good reason for it, like maybe they are spammers and their account was never activated after they registered. If you activate them, you may start getting spam on your board. Maybe you should delete them instead? Or you could send them reminders that they need to activate their account.
In any case, go to the Inactive users page. Here you can select multiple users and choose from the dropdown whether to activate, delete or remind.
Have you hugged someone today?