If you don't mean by approve all users at once (like in one click) than you can setup moderation as below.
To moderate new user you must have to set option Enable moderation (approval) of users find on Admin > Spams
Once you set than all new user registration email will be sent to the admin and account will be active after your approval. You can approve new registered user from Admin > Approve users page. If there is any user waiting for approval you will have counter on that navigation item. You also can approve from user profile page as well.
If you have not set above option yet, there is not way to approve them (as per my knowledge). since they are already in the system as a approved user.
