If you want o display multiple group in ajax registration form, here is how to do it.

1. Simple way, you can put the following code in your bp-custom.php or your theme’s functions.php

add_filter( 'bpajaxr_xprofile_args', 'bpajaxr_show_multiple_profile_groups' );

function bpajaxr_show_multiple_profile_groups( $args ) {
	
	$args = wp_parse_args( $args );
	
	//set groups
	$args['profile_group_id'] = '1,2,3';//where the values are profile field group id, you must keep 1 as the base group is required
	
	$args = build_query( $args );
	
	return $args;
}

You can change the group ids as you want.

2. Alternative way.

Copy the ajax-register-form.php from bp-ajax-registration plugin to your theme. Now modify it as you want. The plugin will use it.

Hope that helps.