Is there a limit on the number of participants on a conversation?

I started a conversation for those interested in a new product I am attempting to market. I fund out how to add others and did so earlier. Now the 'add others' button is gone.
The thread is Quickchange Side Plate Adaptors. Can you help?

 

there is a limit, something like 5 or maybe 6 unless you're an alliance member. then it's like 10 or 12 if not more. i don't have that many friends. i tend to be an asshole