Shamim's Plugins

It require many code change.
If you need this feature badly you can contact through https://www.shamimsplugins.com/hire/

There are add more buttons for Admins and Groups in Front End PM PRO > Settings > Recipient. Does those work?
Are you having any js error in role to role block setting page? Please see browser console and let me know.

Unchecked Front End PM PRO > Settings > Recipient > Can users send message to admin.
Yours can still send message using https://www.shamimsplugins.com/docs/front-end-pm/shortcode/fep_shortcode_new_message_form/

Add following code in your theme’s (child theme’s if any) functions.php

add_filter( 'fep_get_option', function( $value, $option, $default, $is_default ) {
	if ( 'gm_groups' !== $option ) {
		return $value;
	}
	if ( is_admin() && 'users' !== @get_current_screen()->parent_base ) {
		return $value;
	}
	if ( ! is_admin() && 'settings' !== @$_GET['fepaction'] ) {
		return $value;
	}
	
	// Change this with which role you want to give access to all groups
	$allowed_roles = array( 'editor', 'administrator' );
	
	if ( array_intersect( $allowed_roles, wp_get_current_user()->roles ) ) {
		return $value;
	}
	
	// Change this with your desire groups slug
	$groups = array( 'test1', 'test2' );
	
	foreach ( $groups as $group ) {
		unset( $value[ $group ] );
	}
	
	return $value;
}, 10, 4 );

Change roles and groups as required

From version 10.1.1 this works your way. Users can get only messages which is sent when they are in that group.

Add following code in your theme’s (child theme’s if any) functions.php

add_action( 'set_user_role', function( $user_id, $role, $old_roles ) {
	global $wpdb;
	
	if ( ! defined( 'FEP_PARTICIPANT_TABLE' ) ) {
		return false;
	}
	$mgs_ids = $wpdb->get_col( $wpdb->prepare( "SELECT fep_message_id FROM $wpdb->fep_messagemeta where meta_key = %s AND meta_value = %s", '_fep_participant_roles', $role ) );
	if ( $mgs_ids ) {
		$query = 'INSERT INTO ' . FEP_PARTICIPANT_TABLE . ' (mgs_id, mgs_participant) VALUES ';
		
		foreach ( $mgs_ids as $mgs_id ) {
			$values[] = $mgs_id;
			$values[] = $user_id;
			
			$place_holders[] = '(%d, %d)';
		}
		$query .= implode( ', ', $place_holders );
		$wpdb->query( $wpdb->prepare( $query, $values ) );
		
		delete_user_meta( $user_id, '_fep_user_announcement_count' );
	}
}, 10, 3 );

May be next week.

Thank you.
I really appreciate.