set shopper group by some criterias

[Filu_82]

For one customer I need to do a customization in VM 2. I need to set the default shopper group depending on the URL the user opened. So if the user comes over the .ch domain he will be switched to the CH_Customers Group and if he enters over the .de domain he is switched to the DE_Customers group. This process should be done at the first page so he will see prices depending on the customer group and not just after the registration.

I'm familiar with PHP and it is not a problem to find the URL but i have no idea how to switch the user to an customer group without an registration.

I would be happy for every line of php code

Thanks