How do you join private giveaways?

You need to know the exact address of the giveaway page (i.e., the gifter must tell you the address), and depending on the description, some additional criteria may apply.

Giveaway's author should give direct link to you.

example

Think of a private Giveaway as a Party the guy nextdoor is having.

First things first? Were you invited? No? Then you can't join it. Where you invited? Then you probably got an invitation with the time, date of the party, in this case, you'd had gotten a Direct URL link to it.

Additionally, the party may have rules! You may need to dress a certain way, or bring a hot dish with you. Private giveaways may have rules, such as being a member of a certain steam group, and so on.

Long story short, if you don't know about it, you can't. and that's by design.

FFS, use the search function.

Come on, a new topic about this appears every day! Could've read the faq or searched the forum.

I see a chance to advertise my giveaway! http://www.steamgifts.com/forum/VSiQY/private-giveaway-borderlands