There is a Virus/Hack going around that is using the Steam Friends
list to Hijack your accounts.
It starts off with you getting an invite from someone that is in youre friends list
that is sending you an invite to joins Steam Group Community.
The URL will end with STEAMCOMMUNITY.TK

DO NOT ACCEPT THIS INVITE

If you do, your account get's hijacked - you will need to go here:
https://support.steampowered.com/kb_article.php?ref=2347-QDFN-4366

There are already dozens of threads in the SteamPowered forums on their issue, and
looks like Valve/Steam already have their hands full.
If you get that link from someone in your friends list - email them directly if you can to alert them.
From what I've been reading - It's been taking steam's support a week plus to get back to the people who's accounts have been jacked.

Anyways - Here's your PSA - you've been warned.