Need Advice: how to collect 1 (or more) published messages, "key" value in body

I have a scenario with

• 1 or more messages being Published to the Bus
• at different times
• each of the messages contain a key value that is identical across them

How do I create a process/Subscriber to

• wait for all messages with same key to appear
• collect them to be combined/merged

1. Any thoughts on any existing Workflow or Business Process features that can do this out of the box?
2. How can I create a count/loop that knows when to stop? 
   1. The messages should all arrive within seconds of each other
   2. Is there a way to have a Subscriber capture and count the messages and then send them to another process that gets the count and merges based on the count?

I suspect I will need to use the correlated workflow some how.

Thanks for any advice on existing patterns/features.  It's a similar pattern to event processing with rules where an event triggers a process that applies rules which instruct it to look for and wait for another event, then act when it has arrived -- this is kinda like that.

Thank you!