This means you already have a free account on Circle.
If you are accessing on your computer browser, click on the invitation link again and instead of signing up again, click on the Sign In link at the bottom of the sign up form.
Sign in as usual and you will get access to the membership spaces.
If you are accessing the space on your mobile phone browser, try the downloading the Circle App first as it is a more stable experience. After you have the app, access the invitation link again and it will open up the app for you to proceed.
On the invitation link, you may sign-in again with your existing account. Or if you don't remember your password, click “Forgot Password” to reset your password, then sign in again through the membership link.
Or you can sign in first and then click on the invitation link to gain access to the spaces.