I've created two customer groups called "Vendor" and "PRO". Users from these two groups should also be able to login from Magento Front End. My problem is only users from "General" group is able to login. For all others users, I'm getting error like "Invalid login or password". Though my login credentials.
How have you created the new customer groups & customers? Manually, through backoffice? Or have you used any script?
If it's the first case, then check the website you have associated that new customers, as they will probably be associated to the admin site, which shouldn't be the case
Make sure mcrypt is working. Change the password of a testing account that is in one of the customer groups in the admin/backend.
Try login in with the testing account, if this works try resetting the password with the "Forgot your password?" link on the customer login page.
If you don't have any succes:
Try turning of all the plugin, end retry resetting the password. just to make sure that there is no conflict with one of them. Check your php error log.