Document HEOS groups (#20428)

source/_integrations/heos.markdown

@@ -21,6 +21,7 @@ The HEOS integration adds support for [HEOS](https://www.denon.com/en-gb/shop/am
 - Control play mode (play, pause, stop, next and previous), volume, mute and shuffle
 - Clear playlist
 - Select source from device physical inputs and HEOS favorites
+- Create HEOS groups
 
 {% include integrations/config_flow.md %}
 
@@ -114,9 +115,33 @@ media_content_id: "http://path.to/stream.mp3"
 | `media_content_type`   | no       | Set to the value `url`
 | `media_content_id`     | no       | The full URL to the stream
 
+### Service `media_player.join`
+
+For grouping HEOS media players together for synchronous playback you can use the `media_player.join` service. With the example service data payload down below you'll expand playback of `media_player.office` to the `media_player.kitchen` and `media_player.bathroom` players. Please note that all of the media players need to be HEOS players.
+
+```yaml
+entity_id: media_player.office
+group_members:
+  - media_player.kitchen
+  - media_player.bathroom
+```
+
+| Service data attribute | Optional | Description |
+| ---------------------- | -------- | ----------- |
+| `entity_id`            | yes      | The media player entity whose playback will be expanded to the players specified in `group_members`.
+| `group_members`        | no       | The player entities which will be synced with the playback from `entity_id`.
+
+
+### Service `media_player.unjoin`
+
+For removing a HEOS player from a group you can use the `media_player.unjoin` service.
+
+| Service data attribute | Optional | Description                                          |
+| ---------------------- | -------- | ---------------------------------------------------- |
+| `entity_id`            |      yes | Unjoin this media player from any player groups.     |
+
 ## Notes
 
-- HEOS groups are not currently supported.
 - Receivers with multiple zones are represented as a single media player. They will be turned on when playback is started, but cannot be turned off by the integration at this time.
 
 ## Troubleshooing