I have a list of groups a user belongs to. Those names do not match the name of

I believe Michael is correct about needing to use a custom plugin to do this as currently described. However, I want to propose an alternative design that could avoid this problem. I realize it might be too late to redesign your app, but just food for thought.

Instead of using custom group attributes, you could store the extra metadata about groups in a database table. You could design processes to insert/update this data using Write to Data Store nodes and Paging Grids. The benefit of a process approach is that you could control who can update the groups without having to give users Designer access and you would have audit capability to know who changed a group.

If the data was in a database, it'd be easy to run a query rule to find all the group ids that matched the list of group attributes you have for a user. You wouldn't need a custom expression plugin for this.

I believe Michael is correct about needing to use a custom plugin to do this as currently described. However, I want to propose an alternative design that could avoid this problem. I realize it might be too late to redesign your app, but just food for thought.

Instead of using custom group attributes, you could store the extra metadata about groups in a database table. You could design processes to insert/update this data using Write to Data Store nodes and Paging Grids. The benefit of a process approach is that you could control who can update the groups without having to give users Designer access and you would have audit capability to know who changed a group.

If the data was in a database, it'd be easy to run a query rule to find all the group ids that matched the list of group attributes you have for a user. You wouldn't need a custom expression plugin for this.