Okay I have current day figured out using the following code. I have changed the field type from DATETIME to just DATE by the way. Logging the time to the exact second seems redundant.
$curdate = date("Y-m-d");
$dayresult = mysql_query("select * from admin_users where joindate = '$curdate'");
$daycount = mysql_num_rows($dayresult);
echo "Users who have joined today: " . $daycount . "<br>";
Now if I could just use a similar query to select the users joined yesterday instead of today...😕