Can a user add more than one character to a campaign?

I'm trying to add another character to a campaign but I don't see how to do so.

At the bottom of the dashboard, there should be a button called "Join this campaign" (Unless your GM has locked down membership). Essentially, it takes you to
/campaigns/XXXX/join
In your case, the campaign id is 2041, so you can just follow this link and it should work:
http://www.epicwords.com/campaigns/2041/join