Change default customer group at checkout for guest users

I need to change the default customer group at my checkout page to another group. My OC version is 2.1.0.2
The selection at checkout page are the following:

- Register Account
- Guest Checkout
- Returning Customer

At my e-shop, I have 3 different customer groups for two discount types.

Customer Group List
-----------------------------
- I want member card
- I don't want a member card
- I already have a member card

I need "Guest Checkout" and "Register Account" to take as default the selection "I want member card" in order to take the discount of membership ("Returning Customer").

May require an extension. If no extension can be found on the MarketPlace, create a new service request in the Commercial Support section of the forum to get this done as a custom job.

Hello,

Did you ever find a solution to this? I need to do the same thing.

thank you!