Source

File: wp-includes/user.php

function is_user_member_of_blog( $user_id = 0, $blog_id = 0 ) {
	global $wpdb;

	$user_id = (int) $user_id;
	$blog_id = (int) $blog_id;

	if ( empty( $user_id ) ) {
		$user_id = get_current_user_id();
	}

	// Technically not needed, but does save calls to get_site() and get_user_meta()
	// in the event that the function is called when a user isn't logged in.
	if ( empty( $user_id ) ) {
		return false;
	} else {
		$user = get_userdata( $user_id );
		if ( ! $user instanceof WP_User ) {
			return false;
		}
	}

	if ( ! is_multisite() ) {
		return true;
	}

	if ( empty( $blog_id ) ) {
		$blog_id = get_current_blog_id();
	}

	$blog = get_site( $blog_id );

	if ( ! $blog || ! isset( $blog->domain ) || $blog->archived || $blog->spam || $blog->deleted ) {
		return false;
	}

	$keys = get_user_meta( $user_id );
	if ( empty( $keys ) ) {
		return false;
	}

	// No underscore before capabilities in $base_capabilities_key.
	$base_capabilities_key = $wpdb->base_prefix . 'capabilities';
	$site_capabilities_key = $wpdb->base_prefix . $blog_id . '_capabilities';

	if ( isset( $keys[ $base_capabilities_key ] ) && 1 == $blog_id ) {
		return true;
	}

	if ( isset( $keys[ $site_capabilities_key ] ) ) {
		return true;
	}

	return false;
}