We have had to remove the signature from the bot because people were making a lot of new accounts on the forum and leaving spam comments on our thread to reach the signature post limits...
If you want to use the signature please put it on your profile and message one of the admins of our group. We will write you down in the bounty for participating in the signature campaign.
Oh okay, so that's the reason why can't join signature campaign. However, I think you should make an update or make irfan post the statement as the reason on bounty thread, I see lot of people feel confused on how to join it.