Peripheral Acquaintance Management

Some of my contacts have introduced me to their friends. These people are important to track, because they might someday become primary contacts, but for now they are just acquaintances. I'd like the ability to create an "Acquaintance" list that exists within a contact.
For example, my friend Alex introduced me to his friend Marshall. I need to remember Marshall, but Marshall doesn't warrant his own contact. Can I have a list of Alex's friends under his contact, with the ability to tag @marshall in particular events. Then, one day, I'll maybe be able to "Convert Acquaintance to contact" and have all previous interactions with him already tracked.
P.S. I already sent this feedback to Kevin directly. Figured I'd also post it here.