Are you talking about "Submission Receipts" or "Notification" options? You seem to be asking about the latter one but it doesn't have a drop down so I was a little confused by the question.
Here is an FAQ explaining how you can get other people to receive "Notifications": http://forums.adobe.com/docs/DOC-1424
Issue is what the Submission Receipt > Reply to: field. Currently the author’s e-mail address is in the box because I created the form. I really would like someone else to get the notice that a form has been completed and sent to the system. We’re really excited about the service and see a lot of potential for use. However, we’re very new at this and currently still on the learning curve.
We also wanted to know if we’re seized with the original author’s email address. I signed up but would have preferred to have a more generic email address as the “author” on the sign in so that a number of other people can use this.
Although I created the form, I want the reply to be sent to someone else.
Thank you for your quick response.
If you share the form with other people using the Share button in the lower left corner of the UI (choose Add Collaborators on the menu) then those users will show up in the drop down list.
Also, you shouldn't share the username/password of the account with other people. Instead just share the file with others using the share button. They just need a free FormsCentral account to access your form.
You can find the help documentation here: http://help.adobe.com/en_US/formscentral/using/index.html
It gives you a general overview of everything. If you have additional questions you can just post them here and we'll help you out.
There are also FAQs: http://forums.adobe.com/community/formscentral?view=documents
And these posts describe several of the features in detail: