BuddyPress.org

Here we go – easier than I thought it would be to fix this.

Go to bp-core/bp-core-signup.php and find the following line:

$account_details = bp_core_validate_user_signup( $_POST['signup_username'], $_POST['signup_email'] );

Add this line directly below it:

$account_details=strtolower($account_details);

It just transforms usernames typed in multiple cases in the username field to only lowercase letters.