Correct, 10 max in a group in Mesh (6 for Plus w/ delivery confirmations).
I would suggest something more open like how radio “net” chats are set up. Sort of akin to Cruisers Net - folks that sail around the world know that in most harbors, know they can listen at 8am to channel 68 and have a “group chat” with other cruisers within radio range.
Pick a regular date/time (Sundays at 8pm or whatever) that would be broadcast via the FAQ / twitter etc by Gotenna and known to many, location independent, and use Shout messaging to see who you can reach and talk with, share nearby relay node locations on the map, etc, since that will reach anyone in range. If a group develops on a specific topic or a few very active users, then create a group – and without knowing anyone’s GID, you can tap and hold on any message to add that user to your contact list.
And of course you can use Shout messaging anytime to see if anyone else is in range. I plan on leaving my relay node attached to a tablet if bluetooth range allows, so I can read or send shout messages regularly and see who else is in the area.