The go-to answer would be to use the features of the CMS you are developing within for that particular website, and create an account, with unique password, for each person who is authorized to be inside that website as a developer. Users could then put these passwords into a centralized system like DICEware or KeePass, and go anywhere they need to with one master password. The advantage is that you use what you're given. The downside would be having to navigate several CMSes separately for even basic user administration, as well as having to put up with the varying levels of functionality each CMS provides in terms of auditing and content protection. There may also be increased subscriber costs involved in having that many users, both on your side and the clients'.
The second answer would be to develop a centralized system to control access to each of your CMSes; users would log in to this system, which would then act as a pass-through proxy for the editing capabilities of all the websites your teams control by using a single account for each CMS. The upside is one gateway and thus one gatekeeper, and theoretically this system can allow, restrict and log whatever you like regarding user actions. The downside is increased complexity, and the requirement that all portals have the ability to authenticate users in such a way that even the actor using the system can't learn the credentials.
A hybrid approach is authentication using a OpenID provider. Require all users to create a GMail account or similar Google ID for the job, which they'll use to log in to a session. Your websites can then be logged into using Google, identifying the CMS user as the Google user. The upside is one login, one time. The downside is that you still have to go into each CMS and enable or disable accounts, because you don't control access to the Google ID created by the user. You can mitigate this by implementing your own OpenID provider that the websites will look to, based on the user's Active Directory login or some other centralized authentication system.