If you’re not already using it, I highly recommend Atlassian’s Confluence Wiki.
I’ve been using Confluence for over a decade on various projects where there were private Confluence servers (due to issues of confidentiality). For nderground I have an “On Demand” Atlassian subscription for Confluence. It’s only $10 per month for five people. You can create different “spaces” and give access to each space individually.
My philosophy is don’t put it in email, where’s its hard to find. Put it on Confluence where you can share it with your team and where it can be found months in the future.
Confluence can also be integrated with their bug reporting system, Jira. I also use Atlassian’s Bitbucket. I liked the fact that Bitbucket is, by default, private, where as my perception of Github was that it was not.
As you might guess, over the years I’ve become something of an Atlassian fan-boy.