That’s me! Probably best to send any inquiries to support per https://jmp.chat/faq#support as was mentioned earlier - note that I do review all support inquiries so I’ll pickup anything in there that seems related to this thread (though someone else from JMP might reply first).
On the group chat question, to start a group chat it’s as was said: you use the Jabber ID of +1...,+1...,+1...@cheogram.com (numbers separately by commas) for the contact. To answer another sub-question, if you sort them in increasing numeric order, then replies will come back on the same Jabber ID.
I don’t monitor these forums too actively, so best to reach me via JMP support (or email - I’m easy to find). Glad to see so much interest in JMP here!