When joining in the deal, it will ask to select signer. You should be able to select from your existing team member or you can write name and email address who will sign it. Please have a look in this guide:
How to join and negotiate a deal
If you click on the "continue" button in the deal invitation page, it will ask you to provide name and email address to signup. In the next step it will ask to agree with the disclaimer and select the signer. Once you select signer and continue, it will show you a form where you can fill necessary information to continue with the deal.
You can always check our user guide at - http://support.trakti.com