I would do the following:
1. Write the "interested" groups to a user-defined property. This can be done by comparing the user's groups against a list, then using the "List.LastMatches" property to determine which items are present in the user's groups.
2. In order to write the *list* of "interested" groups, you must cast it from a list of strings to a string.
I'm trying it right now and it doesn't seem to work properly, I've got some fine tuning to do, but this is a good base. I'll post once I get it working.
I had a question on this recently and looked back at my old post. It never did work like you mentioned. I found a way to get this working quite nicley.
All that is required is that you populate the "Interested Groups" with your interested groups, and then the users groups will be widdled down to only the groups you care about.
Attached is a revised example of this in case anyone needs it
Filter Groups.zip 970 bytes