"IM" section in contacts

[ColinZack]

I've always noticed there was an "IM" section on some of my contacts whom I chatted with on gmail or gtalk, but there are some contacts it seems without it who I also talk with on gtalk. All the ones who have the "IM" section on their contact page have a green or orange dot by their name to signal their gtalk status. I was just wondering if anyone knew how this works and how to add the "IM" section to someone's contact info since I can't figure it out. I have the persons gmail account listed and as the default email, but still no "IM" section.