We are playin a campaign and we have a person who will be joining us for a special night, are we able to bring this character in to the campaign without having to recreate her character in the campaign? I have the character on my DDB account.

Add the character as normal, then later remove the character. Done.

Step by step what DxJxC said:

- go to the campaign, copy the join link at the top

- paste that link into your browser

- choose the character you want to join the campaign with

It's a bit confusing since the character page itself doesn't have the option to join a campaign. :-)