synapse/docs/admin_api/room_membership.md

# Edit Room Membership API

This API allows an administrator to join a user account with a given `user_id`
to a room with a given `room_id_or_alias`. You can only modify the membership of
local users. The server administrator must be in the room and have permission to
invite users.

To use it, you will need to authenticate by providing an `access_token`
for a server admin: see [Admin API](../usage/administration/admin_api/).

## Parameters

The following parameters are available:

* `user_id` - Fully qualified user: for example, `@user:server.com`.
* `room_id_or_alias` - The room identifier or alias to join: for example,
  `!636q39766251:server.com`.

## Usage

```
POST /_synapse/admin/v1/join/<room_id_or_alias>

{
  "user_id": "@user:server.com"
}
```

Response:

```json
{
  "room_id": "!636q39766251:server.com"
}
```