Change the word "Seat(s)" to "Member(s)" for Paid Memberships Pro Sponsored Members Add On.

my-pmpro-change-seat-to-member.php

<?php
/**
 * This will change all instances of seat(s) to member(s) for the Sponsored Members Add On - https://www.paidmembershipspro.com/add-ons/pmpro-sponsored-members/
 * Add the code below to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function my_pmpro_change_seat_to_member( $output_text, $input_text, $domain ) {
	if ( 'pmpro-sponsored-members' === $domain ) {
		$output_text = str_replace( 'seats', 'members', $output_text );
		$output_text = str_replace( 'Seats', 'Members', $output_text );
		$output_text = str_replace( 'seat', 'member', $output_text );
	}
	return $output_text;
}
add_filter( 'gettext', 'my_pmpro_change_seat_to_member', 10, 3 );

need to change domain to pmpro-sponsored-members

Thanks @kimwhite, this has been updated :)