User story:
I sometimes need to contact more than one person via chat, e.g. for contact 2 people that share a role.
I suggest to add a message creation button to the popup that opens when you click on the icon “Messages”. When you click on this message creation button, you see the sidebar opening with two fields: The first field asks for the recipients and supports autocomplete based on usernames and total names. The second field is the text field we know from chats.
I’ve had a similar experience and need! But there’s a background story to why this has not yet been implemented (not meaning that it cannot be implemented!)… I’m not sure I can explain, but I’ll try it anyway. 
Apparently people experienced from Foodsharing and Yunity times that private group messages provided for the creation of unaccountable and unofficial groups to be formed, intentionally or not. I’ve heard this both from @nick in our last meeting and @tiltec a long time ago, so they can probably tell this it better. It’s a matter of group governance and transparency.
I can see the point there, but I’m not sure these things can be stopped by not offering the feature. It can be made less convenient ofc. There are also some cases in which private group messaging is necessary to the group’s well-being and not necessarily doing the above
I bring these reflections because instead of being a question of either or, I think they can contribute to Karrot being intentionally design for good participatory and transparent group governance. Perhaps we can still have private group messaging that is somehow made visible - only the participants and number of messages, for example. Or should even the content of the messages be visible? Should the group messaging be made possible within teams/subgroups only, so we connect this with the design of a bigger feature?
I’m happy to brainstorm many other possible solutions, while also keeping the more straightforward solution that you suggested, Daniel. I’m interested to hear your and other people’s thoughts!