If we want to start a service like an exchange, I would keep most of the details private and don't discuss them on some free service, which we don't have audited, or so. Nobody knows, who might read our postings there.
There is an important point must be clear that any paid or free services,It cannot be trusted completely ensured.Therefore used the service for any of them because team members meeting and discuss general points of the project.
As for the project's confidential cases It will be exchanged between us by GPG only.
I suggest conducting poll between members of the team to choose one of these services, to accede by all team members.
