You create them yourself Radiant. On your main fellowship page where your teams are listed, under the list of team members (only if you are a captain, I think) you'll see a drop down box with fellowships which are able to be challenged. Presumably if they have enough team members. Then you select the fellowship you'd like to challenge and define your tournament.
The next bit is what I'm confused about! I seem to be able to accept challenges, even when that team has a captain who isn't me. So I'm not sure if the captain can accept them too.