Before I can best point you towards an answer to your question, can you just describe what it is you are wanting the substitute person to be able to do?
Are you just wanting to have a different person be the project manager on a project you currently manage? Or are you an administrator for you organization (with TDAdmin access) and you want to make another person an admin as well while you're gone?
If you can provide that information, I can better answer your questions, or point you to Knowledge Base articles we've written that will answer them. Thank you in advance for any information you can provide.