Hi all,
In JSM there is no out of the box functionality for assignment group. This is standard in almost every other ITSM that I've worked with and it amazes me that this isn't already part of the product.
I've seen a lot of forum posts about how to achieve this with scriptrunner / addons but I've found a way to implement this with standard jira functionality so I thought I'd share this with the community. Apologies if this has been done already, I haven't seen it if so.
- Create a Custom Group Picker field and call it Assignable Group (or whatever you want)
- Go to the project that you want to implement this is in
- Go to Project Settings > Permissions
- Change the permissions in 'Assignable User' to only allow members of Assignable Group

This will only allow you to limit the assignee to a user that exists in the chosen Assignment Group.
THE PROBLEM IS I can't see a way of limiting the group picker field to only include the groups that you want to use as assignment groups, it will list all of the groups that exist in User Management > Groups. Including jira-administrators etc.
To get around this I've had to create a list / cascading list field that only contains the Assignment Group that I want. Assignable Group is then hidden from all screens.
When the user changes the Assignment Group field an automation rule will run to:
- Clear the current assignee (they probably won't be part of the newly assigned group)
- Increment the Assignment Count (which also doesn't get tracked by default...)
- Populate the Assignable Group based on the Assignment Group chosen (using IF/ELSE)

Hoping this will be useful to others. Also, if you can suggest a better / simpler way of achieving this functionality I'd love to hear it.
Many thanks,
Dave