Yes, it's a common fear. I joined a couple in October. We focus on time together --spending time as a group, but also breaking into couples-- to build individual friendships/relationships. My couple is long term poly; they make an effort to provide individual time.
My couple has been together for 17 years, so I was a bit intimidated. They know everything about each other and I was learning about everything from allergies to inside jokes. Their shared history bothered me until I started listening to the stories, and realizing that we would be making our own memories. After only 9 months, we've got some amazing memories - a Broadway play, quiet times in upscale restaurants, shopping adventures, even buying furniture together.
Welcome to this wonderful journey.