I'll try to answer the best I can!
Membership isn't required for anything here in the group! It's all up to you wither you want to be one or not~
As for the Challenge folder, they are specifically for the Bi-Weekly Art Challenges and are open for a week after the Challenge starts. Each Challenge has specifications for what will be accepted. After that week, there's another week for everyone to rest before the next Challenge starts.
The only folder that is open for submissions at any point is the Glowup folder which has it's own very specific rules for submission. I hope that answers your question!