I'm wondering if there is an easy way to either start a conversation to ALL users, or alternatively (and preffered) any way of starting a conversation with all ACTIVE users (i.e. those that have logged in over the last 30 days for example)??
The only way you could do that would be to search for the users in the ACP, based on the criteria you want.
Export the list (copy and paste).
Then extract the user names and add them to the conversation.