Not really sure what you mean, since a customer doesn't become a member of a customer group until they register., So, whatever customer group you want them to be a member of should be set as the default in your store settings (under the "Option" tab) in the admin.
Modules for OpenCart 184.108.40.206
Homepage Module [Free - since OpenCart 0.7.7]
Store Manager Multi-Vendor/Multi-Store management tool
If you're not living on the edge ... you're taking up too much space!