In order to counteract spam and phishing accounts on Steam, Valve has introduced Limited User Accounts. These are user accounts that have spent less than $5 USD on Steam. If you happen to have a Steam account but have never spent money or only received games as gifts from friends you’re following unfortunately this means you won’t have some key features on Steam.
These restricted features include:

Sending friend invites
Opening group chat
Voting on Greenlight, Steam Reviews and Workshop items
Participating in the Steam Market
Posting frequently in the Steam Discussions
Gaining Steam Profile Levels (Locked to level 0) and Trading Cards
Submitting content on the Steam Workshop
Posting in an item’s Steam Workshop Discussions
Accessing the Steam Web API
Using browser and mobile chat

To escape being a Limited User Account all you have to do is a add $5 USD or more to your wallet, purchase a game that is $5 USD or more, add a Steam Wallet card to your account or purchase a Steam gift that is $5 USD or more.
The usage of USD is important as Steam will convert whatever other type of money that is used in your region into USD using the daily exchange rate. Activating a retail game on Steam, receiving a gift, playing demos or free games, and participating in promotional events such as Free Weekends will not activate your account.

Posted: Apr 18, 2015 6:58 pm
Related Forum: PC Gaming Forum

Source: http://www.dualshockers.com/2015/04/18/valve-introduces-limited-user-accounts-restricting-those-who-have-not-yet-spent-5/