Forum Discussion
Adding an external user as MCAS administrator
Not yet, I guess there are some additional requirements that are not documented. We will open a ticket with Microsoft. We have no issue with others like Azure ATP, Defender ATP, Security Center etc. but then they are totally different products.
Interesting. Might try this myself and see what results I get. Please will you post back if you get a resolution in the meantime? 
PeterRising Sure, will do. I've also sent you pm if you want to try it with our test environment.
I just tried this out between my two test tenants and it worked great first time.
Under Settings > Admin Access, I added Cloud Discovery Global Admin access;
Then on my other tenant (to whom I sent the invite), from the dropdown I was able to select the tenant I'd sent the invite from;
Guessing this is exactly what you also did?
Yes, those were the steps but I can't see the client tenant when logging into our tenant (from the account dropdown list). We'll have to check with the client again, maybe they did something wrong though they said they followed the instructions to the letter. Normally, one would get an error if the account they tried to add had some sort of problem.
One thing that maybe has some importance is the fact that my account has been set as "guest" into the client's Azure AD (to perform other administration tasks) but I would think that should not affect the ability to add the same account as external admin in MCAS.