you can claim without "signing up" in the collection. ao3 has some confusing language when it comes to prompt meme challenges.

signing up for the fest: filling out our writer sign-up google form
signing up for the ao3 challenge: ao3 calls making a prompt/request "signing up". we just call it "prompting" for this fest
joining the ao3 challenge: publicly adds you to the people page of the challenge. functionally irrelevant

you can claim prompts without "signing up" (requesting prompts of your own) or joining the ao3 challenge. hopefully that makes sense! they didn't choose the most intuitive language, to put it lightly haha