How Can Strangers Get On My Private Giveaway?

They are 1337 h4ck3rs

Lockpicks.

Probably some of your community members just gave the URL to someone.

Someone linked it.

How well do you trust members of that other community?

Once I went into a "foreign" private giveaway. Reason: I saw a group and their announcement was like: "Private giveaway here". Clicked it and was in. Then I left the page, because a wild horde of guys would have killed me, if I would have left a commend, I guess.
Can happen, but then, because someone is negligently. Or hackers. Or aliens.

They found the link in pastebin

Ask for a reroll and only send the link to those that entered in the community page

It spreads exponentially.

Give me the link, and I'll tell you where they found it.

:)

www.google.com: steamgifts.com/giveaway/*

Probably bruteforced? There are only 844596301 possabilities for the giveaway code ;D

Those "strangers" are actually hackers from the deep web.

someone leaked it. if they win reroll.

Oh well, now since it's not private anymore and yet the people in the community didnt even gave an F about I mind as well turn it into a puzzle.

The only way to keep it private - post giveaway code like that: QQQQQxQQQQ9QQQQ1QQQQsQQQQVQQQQ lolol.
Or reroll it till someone from your group/friends wins.

Put in the giveaway description that you have to be part of the community to join, if someone wins it but doesnt meet the qualifications you just ask support for a reroll and someone else will win it.

Just for the future, use pastebin or translate the url into binary or base64 and such things...when someone google it he will not find it so fast...