I can't add a member to my Board and I have a premium account

[Tim Martinez]

I get a message that says that I have to add the person to my workspace. When all I want to do is add them to a board. I have 13 members on that Board and need to add more and my understanding is that I can have as many as I want. 

[Nils Geylen]

First what comes to mind is "is this board in your paid Workspace?" 

Because it's technically not the account that's Premium but the space. People can own or be on any board, on any of a number of spaces, free or paid.

Or, could this board be in someone else's workspace that you're a member of?

Second is, in Workspace Settings, is Sharing boards with guests set to Anybody? Maybe you turned that off at some point? Like did you add the other members just minutes earlier and did it just stop working, or did that happen some time ago? That might tell us something.

And also under settings, have you restricted members to be on specific email domains?

Just a couple of guesses, coz I've never seen that happen, and we add dozens of observers on a rolling basis all the time.

So yeah, on Premium you should not have any realistic limit to the member count afaik.