You may only join one program at a time. In order to join a different program, you must first leave the program that you have currently joined.

You can leave a program by following the steps below:

Logging in to your account via grokker.com
Go to the program page
On the far right side, scroll down to under "Related Programs".
There will be a box that says "Changed your mind? Leave Program" and choose "Leave Program".
Once you have left the program you are able to join a new one.