It is very easy to "join" a website using some one else's details, or even totally fictitious details. (Unless credit card details are required, and even that can be achieved)

At a previous company I worked for we had a problem with disgruntled employees registering their manager's business e-mail to receive porn mail.

There is very little that can be done to prevent this kind of situation evolving that is why evidence of this nature would need to be backed up with certain proof that the person concerned actually submitted these details, and actively connected to and used the site.
This proof would have to be gathered from the PC concerned, the site concerned, and the business responsible for the connection i.e. the user's ISP connection logs. Currently this information can only be collected after a warrant has be issued to the appropriate law enforcement agencies

I agree with -DaRK-RaiDeR- in that you would need to supply further information regarding "who" and "why" and what the accusations are.

Golam