This is not possible on FormsCentral but as you might already know FormsCentral is retiring soon. So, you will probably soon need to move your forms to a service like JotForm. Is this an important criteria for you when you decide which Form Builder you will move your forms to? If so, you might be happy to know that JotForm has this feature. It is called "sub-users". You can give other people access to edit your forms.
Actually this is possible assuming the folks you want to add have Formscentral accounts of their own. They can be free accounts but they have to have their own account.