what is group message ?

Can't say that I have tried it. My vague understanding is that this similar to a group text in regular SMS. Everybody in the group sees the original message, and everybody sees each other's responses.

This is explained (somewhat) in this KB article:

https://support.garmin.com/en-US/?faq=icTGocuFYw88BFNWburNv8&productID=893837&searchQuery=group&tab=topics&textPage=1

You have to dig down into the individual "sending a message" topics to see what passes for an explanation. As far as I can tell, a message becomes a group message when it has multiple cell number recipients. It seems odd to me that this is limited to cell recipients, but maybe that makes sense?

skyeye The group messaging feature allows you to set up a group message to multiple contacts and message back and forth with all of them. It looks and acts similar to the iphone group messaging. Frequently Asked Questions About inReach Message Charges

Is there a limit of contacts for a goup message (5?). And the user should keep in mind, that a sent group message only counts as one message, but each reply will count towards your message limit. 

sending a message to a number of recipients was always possible. And when each recipient did replay, this is also normal.

But here as far as I do understand it, this replay to the original message will be copied to all original recipients of the original message.

If this goes over Iridium, then this could produce really lot of messages within minutes.

I still hope, I did not understand all correctly as far as those group msg concerned.

Also have to consider message charges to participants. In the classic iR way of charging for iR to iR messages, both the sender AND the recipient are charged for the message. If that scheme carries through here, a group chat is going to generate a LOT of charges. If the group has N participants, each message to the group (new message or reply) will result in N-1 messages charged to the sender, plus 1 message charged to each to the N-1 recipients. 

I vaguely remember some discussion about possibly NOT charging for some messages. It was never clear to me if that was to apply to things which happened to be delivered via Internet, or some other scheme. I recall thinking at the time that it did not sound like charging was well-thought-out.

Just a question: is group messaging only working via the app or does it work also directly from the messenger?

so far I do not see any difference. I can send a message to multiple recipints on the device same as on any other inreach units.

The same can be done from the app.

One thing seems to be not existing on the messenger itself is the 'new post'

There is only a new message so far.

That was in fact also my orignal Q, what exactly is a group message?

If this is simply a message with multiple recipnts, then this can be done from the device as well.

Are you sure? I thought, if I sent a message to multi recipints from the messenger, they all get my message, but if one answered, just me is getting the answer message? But maybe I am wrong...