You need to set the $sso_params_array inside of the foreach loop. Something like:
$myusers = get_users();
foreach ( $myusers as $myuser ) {
$sso_params_array = [];
$sso_params_array['custom.user_field_1'] = $myuser->user_login;
$sso_params_array['custom.user_field_2'] = get_user_meta( $myuser->ID, 'first_name', true );
$sso_params_array['custom.user_field_3'] = get_user_meta( $myuser->ID, 'last_name', true );
WPDiscourse\Utilities\Utilities::sync_sso_record($sso_params_array, $myuser->ID;);
}
Try it with just one user first though. For example:
$myuser = get_user_by('id', 1);
It might be best to create a test user for this. You can get the user with this function: get_user_by() – Function | Developer.WordPress.org