How to stop "Request Participant" field from resetting when it is updated using automation rule?

[IT Brute]

I have an automation rule that adds the users mentioned in the comments to "Request Participant" field. 
It works fine only once. If the user is mentioned again or any other user is mentioned the field is reset and the users are erased from the field. 

The question is how to append values to the "Request Participant" field so that users mentioned in the comments are added to the existing users in that field. 

Another method I tried was taking the existing user name in this field, store it in a variable. Add the values from this variable and the variable that stores the user ids of the users from the comments and add the two the fields. This is somewhat successful as all the participants now get a notification that they were added to the work item. Not an ideal solution. 

The project is Team-managed Service Management. We are on Jira Cloud. 

This is the flow of the rule: 

1. When: Rule is triggered on - All Comments

2. Then: Create variable - Create smart value variable {{adduser}}

2.1 Smart value - {{issue.comments.last.body.match("accountid:(.*?)\\]").join(",").remove("accountid:").remove("[~").remove("]")}}

3. And: Edit Work Item Fields 

3.1 Choose Field - Request Participant

3.2 UserInComments - {{addUser}}

Any help is appreciated.

Thanks! 

[Kai Becker]

Hi @IT Brute 

you should be able to solve this with the additional fields part in the edit work item action.

This allows you to add users to a multi user picker field. Please see the second code block unter Mulituser picker: https://support.atlassian.com/cloud-automation/docs/advanced-field-editing-using-json/#Multi-user-picker-custom-field

[Mikael Sandberg]

Here is another good KB on how to handle this, https://support.atlassian.com/jira/kb/add-update-remove-request-participants-using-automation-in-jsm-cloud/