Hi Marie. Typically, there's no "sign-up", really. When you post an entry for a challenge, you include the tag associated with that challenge. For example, there's a "Mundane Vegetable" challenge that was just announced. To include your entries, tag them with mundane-vegetable. That's really all there is to it!
http://365project.org/discuss/themes-competitions/22150/get-pushed-week-92-monday-april-21st-to-sunday-april-27th-repost