Starting with version 9 of Adobe Acrobat/Adobe Reader you MUST have the digital signature fields in the document first. If you don't Acrobat 9.x and later lock the document from further editing so you will not be able to add more digital signatures without a pre-existing digital signature field. Acrobat 8 and earlier did not require this.
Let's start with which version of Acrobat Pro are you using? Next, tell me a little about the PDF file you are trying to sign, does it have existing signature fields and if you select the File > Properties menu item , then select the Security tab are there any restrictions? Finally, since you said "we are having" I presume there are multiple users in play, are they all using Acrobat or are some of them using Reader?