I've told you about Giftie Etcetera's blog; her Facebook group is a fun place to continue the conversations that began on her blog. I love the opportunity to communicate with like-minded planners.
Another favorite is DIY Planners & Organisers. It's main focus is the Do It Yourself aspect of creating your own planners, whether it's from making your own inserts or finding them online to create a custom planner that works for you.
I recommend both of these groups especially to those who are beginning new planners. Don't be shy - jump in and ask questions - there are lots of lovely people on these groups who are willing to share their knowledge with you!