Restrict groups for Group picker custom field

[Davide Cioranu]

Hi all,
I've looked thoroughly through the community but I couldn't find an answer specific to JSM.
I'm currently trying to setup a Custom field Group picker.
I've defined the default group to be selected, but after configuring the field in the request, the field allows me to pick between all the Groups available in Jira, which I believe it's its usual behavior. 
My goal is to have only a specific set of groups to be picked from.
Is there a way around this? 
Can groups be restricted from Projects so these won't appear in this field?

Thanks

[Mikael Sandberg]

No, you cannot limit the group picker field to a subset of groups like you can with a user picker field. 

[Mikael Sandberg]

You would need an app to work around it, I believe Scriptrunner can do this for you.

[Davide Cioranu]

How would it work then for any Service desk with multiple teams/group within it? 
Let's say there is a L1, L2 and L3 group, and each one has got 3 members, what is the best practice when setting up a service desk with these, each one with their queue of issues?
My goal is to have a group field linked to the groups in Jira, and the "Assignee" field would be displaying the choice between the members of the respective group.
Example:
L1 : user A, B, C
L2 : user X, Y, Z
L3 : user 1 , 2, 3

can this be only done with scripting? 
I believe I'm not the first one using Jira for a Service Desk, that's why I was interested in this

[Mikael Sandberg]

So my team is divided like that and what we use is a custom field called tier that is set based on the assignee. We also have an automation that auto-assign new requests based on a on-call schedule in Opsgenie. 

[Davide Cioranu]

What type of custom field would it be? A group picker? 
I can't find any custom field which give me the option to link a value within it to an agent, unless that's not the work it is meant to be setup.

[Mikael Sandberg]

The custom field we are using is just a single select field. I then have an automation that sets that field based on the assignee.