I am using the pickerFieldUsers component on 7.5 to display all users from group

You would need to create a group that contains these users. Have you looked into creating a rules-based group that includes the users you want?

Alternately, create a custom picker instead : forum.appian.com/.../SAIL_Components and base it off of the list of distinct users that you want to be able to pick from.

I was using a custom picker originally, binding it to an array of users returned from this rule (GroupB is a subset of GroupA):

difference(
getgroupmemberuserspaging(cons!GroupA, 0, 2500),
getgroupmemberuserspaging(cons!GroupB, 0, 2500)
)

It works fine when there aren't many users (few hundreds), but on environments where there are thousands of users, the performance is very slow during loading of the page and when searching for users. That is why I wanted to switch to using the new pickerFieldUsers component.

I have looked into creating a rule-based group, but I don't think there are membership rules in place to mimic what I want to do up there. Unless I am missing something? Any suggestions?

Thanks!

Instead of rules-based groups, perhaps you should just create this as a regular group, and manually set the members in them when you create user accounts? If you create users in a process, you could add them to this new group when they need to be. Alternately you could create a utility process which runs off a timer and determines group membership and adds them as needed.