Help with a simple SQL queries.

I need to add the new Users ID that was added to the database in a email.
The code is already done, I just need someone to make the SQL that gets the last MAX userID form table USERS and adds + 1, and puts that in the line below

$email_message .= "firstname: ".clean_string($_POST['firstname'])."\n";
$email_message .= "lastname: ".clean_string($_POST['lastname'])."\n";
$email_message .= "username: ".clean_string($_POST['username'])."\n";
$email_message .= "email: ".clean_string($_POST['email'])."\n";
$email_message .= "street1: ".clean_string($_POST['street1'])."\n";
$email_message .= "street2: ".clean_string($_POST['street2'])."\n";
$email_message .= "suburb: ".clean_string($_POST['suburb'])."\n";
$email_message .= "state: ".clean_string($_POST['state'])."\n";
$email_message .= "postcode: ".clean_string($_POST['postcode'])."\n";
$email_message .= "country: ".clean_string($_POST['country'])."\n";

+++=========================== NEED TO ADD THE BELOW ====
$email_message .= "userid: ".clean_string($_POST['userid'])."\n";
[ADD the new "userid" to email here] Eg Get MAX userID from table "userid" and add +1]
+++===========================

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