I wouldn't do an array because you're not dealing with a standard drop down with a user select field. First I'd swap them over just for a better user experience. ie choose team first then user. The user select doesn't populate on the screen until you click on the magnifying glass or start typing but the list of names is preloaded into the DOM. It's that inbuilt filter on the user select that you need to hook into. I'm not in front of my computer at the mo but I'll look later.

What version of CRM? 

I am on CRM 2019 R2

why I want to do something like this is because, some user should not be able to see anything from another team (team that is not in his display right).

But when the user is creating a case, he is still able to choose user from another team, which I don't want. So I want him to only choose user from his own team and the team that he have display right only.

The team select is just a drop down. If you have a "Support" team or something like that you can use the Search SQL on the assigned user to filter the field to a specific team but set it in the create script on the relevant screen.

The team select is just a drop down. If you have a "Support" team or something like that you can use the Search SQL on the assigned user to filter the field to a specific team but set it in the create script on the relevant screen.