1) As this is a website clique you are required to have a website if you wish to join. Your site can be about anything, although social networking sites such as twitter and facebook are not accepted. Sites such as Livejournal and Deviant Art are acceptable.
2) You should link back somewhere on your site. Although I won't make a point of reinforcing this rule (I don't have the time :P), linking back helps to spread the word and encourages people to learn about invisible illnesses. Since that is the purpose of the clique I hope you will consider linking back :)
3) Direct linking of any image on their site is strictly prohibited and will result in your membership being rejected.