To unblock a certain user, go to your admin dashboard and find the menu on your left. Click "Limit Attempts" -->“Blocked” and hover your mouse over the necessary address in the “IP Address” column. Click "Reset block" caption and the IP address will be unblocked. Use “Bulk Actions” to unblock a group of users: mark the addresses that have to be unblocked, choose the “Reset block” action and click “Apply”.
Click "Add to Whitelist" caption and the IP address will be whitelisted.