I just joined the SCP Foundation, and I'm quite happy to have joined. Can't wait to start writing!
If you're interested in writing, do familiarize yourself with the resources available to authors in the Guide Hub, and consider poking around the forums a bit to see how detailed feedback is given. Remember to create a sandbox page for yourself, and get any drafts you write up there shown to the IRC chat and/or the drafts forums for feedback prior to a mainsite post.
You can also get your SCP ideas looked at in the Ideas and Brainstorming forum before you start a draft—there are thousands of articles on the site, and it can help the drafting process a lot if you make sure your concept is unique and engaging before committing to writing it all out!
If you have any questions about the site, feel free to message a staff member.
I hope you enjoy your time here. Welcome to the site!