I don't see anything that allows adding additional participants to a conversation. Where do I need to look in order to do this? I couldn't find out where.
Hmm, I don't see that at the bottom of the participants. No wonder I did not find anything. I ran Firefox ESR in safe mode (extensions disabled) and that made no difference. Also tried the other forum styles. OOOPs.... I was convinced I had started the conversation, but I just checked and errr, welll Sorry for the false alarm.
Are you the conversation starter? Go to "Conversations" and look at the participants. If you are listed first, then you are the "Conversation starter" (you can hover over the name with the mouse and a tooltip should appear)
In addition you should see "Edit Conversation" if you are the Conversation starter: Some things worth to mention: If the conversation starter has selected the first option (see below), anyone can invite others. And in the case of the second option the conversation has been locked and not even the conversation starter can invite others.