First thing first don't use the same account which you used for apple corporate account which is used for build internal apple apps.
because "itunes connect" will confuse later.
I suggest you to create a new apple account(ID), and then login to this apple account and ask your colleague to send you an invitation.
the invitation doesn't have to be sent to the same email you are using for the new apple account.
once you get the email, copy the link and access the link on the same browser which you already logged in with the new apple account.
Then it will tell you the invitation and click please.
try to access " itunes connect"
Then you will be redirect to a new page, if the page is keep loading, that means you have accept "itune connect" with your account.
then just wait for a email.
then you should be able to access
However, in order to access sandbox, you need to be at least an app manager: ask admin to change it.