Opened a Form in Reader. Security feature won't let me sign.

I have an unpaid (30 day trial) version of Acrobat XI, which includes use of Forms Central.

I created a Form in Forms Central, and saved it to my computer as a PDF.

I opened it in Reader, and tried to sign it.

A screen notice told me "Security settings prevent adding a signature to this document."

My Goal: create a Form in Forms Central, save it on my computer as a PDF, share it with my team via e-mail, and ask them to sign it and return it.

Thanks for your help!