## Add

**post** `/v1/express/{expressCustomerId}/users`

Associates an existing user with a customer, granting them access to customer-specific resources and studies.

### Path Parameters

- `expressCustomerId: string`

  Unique Express customer identifier. Format: cus_{32-hex-chars}

### Body Parameters

- `userId: string`

  User ID to add to the Express customer. Format: usr_{32-hex-chars}

### Returns

- `success: boolean`

- `message: optional string`

### Example

```http
curl https://api.avarasoftware.com/v1/express/$EXPRESS_CUSTOMER_ID/users \
    -H 'Content-Type: application/json' \
    -H "Authorization: Bearer $AVARA_API_KEY" \
    -d '{
          "userId": "usr_1234567890abcdef1234567890abcdef"
        }'
```