Create Slack channel for incident action only adds Responders as members

Issue Summary

The automation action "Create Slack channel for incident" won't add selected users as channel members if they're not Responders of that incident.

Steps to Reproduce

1. Enable Incident Management in your service project (Project settings > Features)
2. Create an automation rule using the "Create Slack channel for incident" action
3. In the 'Channel members' field, include users that should automatically be added to the Slack channel

Expected Results

A Slack channel is created and the users selected as channel members are added to it

Actual Results

The channel is created successfully, but channel members are only added if they're also Responder of the incident

Workaround

Add users as Responders before triggering the channel creation