I think you have missed part of the ansible concept, and is mixing up where you loop through the variables. It sounds like you want to call a program that is looping through something you mention in a group_var file, whereas the plugin should just take a single parameter, and do its thing atomically with just the set of options that has been explicitly sent to it.
What you need to do, is to write a wrapper (playbook or role, which is using your plugin).
* The way that your plugin gets data, is per command line parameters. Your module will not really know much about the group vars at all. A plugin is not meant to loop over the ansible host file, or your inventory, an array from group_vars or anything like it. That is done in a role or a playbook.
In your example, you should rather use your playbook or role as (just making up an example):
name: do stuff
myPlugin: credential={{ some_variable }} target_url={{ item }}
with_items:
or
name: do stuff
myPlugin: credential={{ some_variable }} target_url={{ item }}
with_items: "{{ some_array_in_my_group_vars }}"
//magnus