diff --git a/includes/activitypub/handler/class-join.php b/includes/activitypub/handler/class-join.php
new file mode 100644
index 0000000..03c5dc1
--- /dev/null
+++ b/includes/activitypub/handler/class-join.php
@@ -0,0 +1,137 @@
+<?php
+/**
+ * Join handler file.
+ *
+ * @package ActivityPub_Event_Bridge
+ * @license AGPL-3.0-or-later
+ */
+
+namespace ActivityPub_Event_Bridge\Activitypub\Handler;
+
+use Activitypub\Activity\Activity;
+use Activitypub\Collection\Actors;
+use Activitypub\Http;
+use Activitypub\Transformer\Factory;
+use ActivityPub_Event_Bridge\Activitypub\Transformer\Event as Event_Transformer;
+
+use function Activitypub\is_same_domain;
+use function Activitypub\object_to_uri;
+
+/**
+ * Handle Join requests.
+ */
+class Join {
+	/**
+	 * Initialize the class, registering WordPress hooks.
+	 */
+	public static function init() {
+		\add_action(
+			'activitypub_inbox_join',
+			array( self::class, 'handle_join' )
+		);
+
+		\add_action(
+			'activitypub_event_bridge_ignore_join',
+			array( self::class, 'send_ignore_response' ),
+			10,
+			4
+		);
+	}
+
+	/**
+	 * Handle ActivityPub "Join" requests.
+	 *
+	 * @param array $activity The activity object.
+	 */
+	public static function handle_follow( $activity ) {
+		$actor = Actors::get_by_resource( $activity['actor'] );
+
+		if ( ! $actor || is_wp_error( $actor ) ) {
+			// If we can not find a user, we can not proceed the join process.
+			return;
+		}
+
+		if ( ! array_key_exists( 'object', $activity ) ) {
+			// If the object is not set, we can not proceed the join process.
+			return;
+		}
+
+		$object_id = object_to_uri( $activity['object'] );
+
+		if ( ! is_same_domain( $object_id ) ) {
+			// If the "Join" object is not owned by this WordPress site, abort.
+			return;
+		}
+
+		$post_id = url_to_postid( $object_id );
+
+		if ( ! $post_id ) {
+			// No post is found for this URL/ID.
+			return;
+		}
+
+		$transformer = Factory::get_transformer( get_post( $post_id ) );
+
+		if ( ! $transformer instanceof Event_Transformer ) {
+			// The target post is not an event post.
+			return;
+		}
+
+		// Pass over to Event plugin specific handler if implemented here.
+		// Until then just send an ignore.
+
+		do_action(
+			'activitypub_event_bridge_ignore_join',
+			$transformer->get_actor_object()->get_id(),
+			$activity,
+		);
+	}
+
+	/**
+	 * Send "Ignore" response.
+	 *
+	 * @param string  $actor            The actors ActivityPub ID which sends the response.
+	 * @param array   $activity_object  The Activity object that gets ignored.
+	 * @param string  $to               The target actor.
+	 */
+	public static function send_ignore_response( $actor, $activity_object, $to = null ) {
+		if ( ! $to && array_key_exists( 'actor', $activity_object ) ) {
+			$to = object_to_uri( $activity_object['actor'] );
+		}
+
+		if ( ! $to ) {
+			return;
+		}
+
+		// Only send minimal data.
+		$activity_object = array_intersect_key(
+			$activity_object,
+			array_flip(
+				array(
+					'id',
+					'type',
+					'actor',
+					'object',
+				)
+			)
+		);
+
+		$to    = Actors::get_by_resource( $to );
+		$actor = Actors::get_by_resource( $actor );
+
+		// Get inbox.
+		$inbox = $to->get_shared_inbox();
+
+		// Send "Ignore" activity.
+		$activity = new Activity();
+		$activity->set_type( 'Ignore' );
+		$activity->set_object( $activity_object );
+		$activity->set_actor( $actor->get_id() );
+		$activity->set_to( $to );
+		$activity->set_id( $actor->get_id() . '#ignore-' . \preg_replace( '~^https?://~', '', $activity_object['id'] ) );
+
+		$activity = $activity->to_json();
+
+		Http::post( $inbox, $activity, $actor->get__id() );
+	}
+}
diff --git a/includes/class-setup.php b/includes/class-setup.php
index 2912002..91140a1 100644
--- a/includes/class-setup.php
+++ b/includes/class-setup.php
@@ -15,6 +15,7 @@ namespace ActivityPub_Event_Bridge;
 // Exit if accessed directly.
 defined( 'ABSPATH' ) || exit; // @codeCoverageIgnore
 
+use ActivityPub_Event_Bridge\ActivityPub\Handler\Join;
 use ActivityPub_Event_Bridge\Admin\Event_Plugin_Admin_Notices;
 use ActivityPub_Event_Bridge\Admin\General_Admin_Notices;
 use ActivityPub_Event_Bridge\Admin\Health_Check;
@@ -187,6 +188,10 @@ class Setup {
 			return;
 		}
 
+		// Register the handler for incoming ActivityPub "Join" activities.
+		add_action( 'init', array( Join::class, 'init' ) );
+
+		// Register the custom ActivityPub transformers.
 		add_filter( 'activitypub_transformer', array( $this, 'register_activitypub_event_transformer' ), 10, 3 );
 	}