To join a challenge, go to the 'Open' section and find a challenge you would like to join.
Mouse over it and press the 'Join Challenge' button.
Select from where you want to upload photos to the challenge.
Once you're done, press 'Submit' and you're in!
Note: You can only submit up to 4 photos depending on the challenge.
Be sure to read the challenge brief and rules before joining to avoid removal.
Currently there is no limit to the number of challenges you can join (as long as you meet the requirements such as status).