Can I build a dynamic group based on part of a value contained in 'MemberOf' user attribute ?

Dynamic group "by query" membership rules can reference membership of groups.  So you can just add the groups that you want to look for members of individually.  You can also do query by container so you could create a Managed Unit called "My Criteria groups" and use a query to pull into it all of the members of groups matching a specific name pattern.  There is risk of some duplication here so experimentation is recommended.

Sorry... might not have been clear above.  You would then specify the "My Criteria Groups" Managed Unit in a "by query" membership rule on your dynamic group.

I looked at this again - you can get what you need just by using an "include group members" membership rule on your dynamic group.

When you go to set it up and you are asked to select the group(s) whose members you want, just type the prefix (Example:  ABC) and when you hit "Check Names" all the groups with that prefix will show up.  You can just select the ones you want or take the whole list.  Either way, you won't have to repeatedly go hunting for them individually.

Thank you, it works well!

Thank you, it works well!