A connection is made once, by a workspace owner, and applies to the whole workspace. Individual members do not connect their own accounts; they use the connection the workspace already has.
Open Settings, then Connections. Each of the three shows either Connect or the name of the account it is currently connected to.
Connecting takes you to the other company's sign-in page, where you approve what Onbrief may do. Onbrief never sees the password you type there. Disconnecting is one click on the same page and takes effect immediately; nothing already brought across is removed.
If a connection stops working, the Connections page says which one and why, and the fix is almost always to connect it again with an account that still has access.