Remember when the Steam community was a smaller and simpler place? Yeah, that time is long gone. Also, Steam groups were either public or private. Not anymore, though.
Apparently, the Steam Team decided that something had to be done to make group signups simpler, now that the community is so vast. According to an official announcement on the Steam Blog, the old approach to invites hadn't kept up with the needs of players. As a result, we got a new group type - the restricted group. For a Steam user to join a restricted group, they have to request permission first. After the request is made, it's up to a group moderator (or the owner) to approve or deny it.
At the same time, the Steam guys decided to take the fight against spammers to the next level. I personally found some recent group invites in my inbox to be nothing less than spammy, plus coming from people I don't know. Fortunately, users can now send invites only to their friends. No more random invites, yay!
The list of new features and improvements to the Steam groups doesn't end here. Here's what the Steam blog has to say about it:
Over the next few weeks, we'll also be working on identifying and banning networks of groups that have been mass-created and exist for the sole purpose of spamming.
We're also working on improvements to the way that friend invites work to make it easier to connect with the people you know and give you more control over invites.