I created a character.

Then I created a campaign.

I can't find the unassign character function.

I've tried sharing the campaign and character to a second account. I've also tried sharing the character to myself - my main account.

But no luck.

Where is the unsassign function, or function to have a character created outside a campaign join a campaign?

At the top of your campaign is the invite link. Copy it and paste it to your web browser. When you open it you will be given the option to create new or add a current character. Pick the character you want to add. Once it's in the campaign you'll have the option to unassign it.

Thanks Sigvard_Vigrid! That worked.