I ain't exactly an expert by I know you can go to admin cp - user - then edit each user manually then click "recieve admin email" but I am sure there may be a quicker way.
Prehaps someone with more knowledge knows, sorry.
You may go to Maintenance > Execute SQL Query > from the dropdown select "Yes - Receive Admin Emails"
But, don't forget that they can just turn it off again in their User CP and you may piss some members off if you set that to Yes when they deliberately set it to No.