The technique for doing this is called "gated forms", and is one of the most powerful techniques in the modern marketer's toolbox. You can set up gated forms yourself to work with your Eloqua marketing database.
Essentially, the technique works as follows:
- if you recognize the visitor based on their cookie, either provide them with the asset directly, or ask them just a little more information
- if you don't recognize them based on their cookie, check to see if they are in your database based on their email address
- if you do not have any information on the visitor at all, ask for a longer amount of information
The following diagram shows how this works:

By extending this simple technique further, you can ask for more information at each interaction, while never asking for an overly large amount of information, and never asking the same information twice.
Gated forms are very flexible as a technique, and can be used in a variety of situations - some as simple as just removing a form in front of a web asset, some as robust as asking for unique information of each visitor at each interaction.
To help you get started, here is a guide showing exactly how gated forms can be created and implemented, from the most simple scenarios to the most robust.