Apparently is impossible to use crowd play as client, no host, using a Youtube brand account. This is a bit annoying as you are forced to use Google main account. That's awful for many reasons.
I know crowd play is currently at beta phase, but I'd like to know this is something is going to change.
If you have any concerns about it, I think it makes sense if you let the Stadia developers know. Especially because they are working on the feature at the moment, they can evaluate your feedback and check this out. Might also be that there is a reason why it's not possible with YouTube Brand accounts, but I think if you do a constructive feedback, that's a good way to let them know.
Just tap your profile picture in the top right corner on stadia.google.com or in the Stadia app and select "feedback" and submit it this way.
Thanks, @Chufu . I've just sent my feedback.