A lot of things go better if more people have more context on the state of a project. Just to name a few:

• Others can point out mistakes
• People can build common knowledge of how much everyone is working
• Others can build better models of your strengths and weaknesses, which means we can coordinate better around those
• If you need help, or become unavailable, others can jump in more easily

Slack is the central place for that kind of context sharing. There is practically no oversharing. Reading is cheap, and even if everyone on the team was continuously writing, I would still be able to at least skim what everyone on the team is saying.

Some specific guidelines we have for this kind of sharing:

• Avoid using DMs. It's OK to use them if you have something sensitive or private to discuss, but the vast majority of your written communication with individuals should be in public channels (either a generic X-Y-dm channel, a project-specific channel, or on their wall channel).
• If you are collecting any kind of analytics, post it regularly to a Slack channel for your project
• CC the team-transparency email on external email communication so that emails show up in in the team-transparency email channel. 

If you are uncertain about how to make progress on a project, a good default activity is to sit down and write 3-4 paragraphs on where your project is at, and what might be good next steps, in your personal Slack channel or project channel. Most likely someone will jump in and help you, or you will have clarified the state of the project enough in your mind to know how to proceed on your own or with your team.

The two most common reasons for why information fails to get shared are confidentiality and spoken communication.

Confidentiality should be avoided at substantial expense. Many external organizations will make frequent requests like "please treat this sensitively" or "don't share this further". Those kinds of requests should always be pushed back on, unless you can identify a good reason for them being there. Many external parties treat requests for confidentiality as cheap and routine. This is usually their loss and a sign of dysfunction, but for that to not affect us, we need to prevent those constraints from reaching into our organizational umbrella.

Spoken conversations on the other hand are crucial and should be the default choice of communication. It's worth the cost of being less in-sync with the whole organization to enable the fluidity and bandwidth that in-person spoken communication enables.

However, in my experience having spoken communication between one team-member and an external party is much more costly than a conversation between two team members. Information and context, including context built through spoken conversations diffuses quickly throughout the organization when two members are both privy to it. But if only one team member is privy to it (because the conversation was with an external party) the default outcome is that the information stays siloed within the relevant team member. As such, we have a strong guideline that important conversations with external stakeholders should have at least two team members present (and those team members are encouraged to either share observations about the conversation with each other in Slack, or to talk about it in a public team space like the team deck).

If an external party asks for someone to keep a secret, ask for at least two people to be shared on the secret (this request might be rejected, in which case it can still be the right choice to accept the secret, just treat it as appropriately costly).