Posting a trade in a group lets other members comment on or copy or even inverse that trade. It's possible to do this from two places in the app - the Positions screen and directly from the group messenger.
1. From the Positions screen
The first place you can post a trade from is your "Positions" page (of course you'll need to make a trade first before you can post it).
To find the trade you want to post, tap the Positions icon in the bottom navigation bar. There you'll see your trade.
From there, tap the three dots at the bottom of the trade. That opens an action sheet where the first option is "Publish to group".
After tapping "Publish to group" a list of the groups you're a member of will appear. Toggle to ones you want to post this trade to on, and then tap "Publish"
That's it, you're done!
2. From inside the group
You can also post a trade directly into a group (when doing this, you'll be posting a new trade not yet made).
To start, go into the group you want to post to and tap the yellow plus icon next to the "Have your say" box.
After that you can trade as you would from the "Trade" icon in the bottom navigation.
Once you've completed the trade, that's it, you're done!
When finished your trade appears in the group.
Then, other members are free to comment on, copy, or inverse that trade.
For more help creating a group, see the full article here.