After receiving invitation for Basecamp on your Gmail please follow the below-mentioned steps:

1. Open https://basecamp.com/ in the browser. 
   
2. Go to Sign-in button.
   

1. Click on Sign-in with Google.
   

1. Enter your Gmail id then Next.
   
2. Enter your password then Next.
   
3. You will be successful in logging in.
   

If any trouble or query please feel free to mail support@growth99.com.