This error message occurs when you try to log in to your Active.com account due to the fact that you are registered internally and have created an online account before accepting an invitation from the camp organization. Below are instructions to resolve this issue:
 
1. Go to http://www.active.com
2. Click Sign In at the upper-right corner to log into your account
3. At the top of the page, hover over your name
4. Click Settings
5. Click Account Settings
6. Change email address to any other email address
Note: If you do not have an alternative email address, please enter anything in the format of an email address (i.e. xxx@123.com)
7. Click Update Email Address

Once this has been completed, your email address is un-linked from Active. You may ask the organization to resend you an invitation link, which will create a new online account for you.