Hi,
unfortunately Joomla allow to assign only one usergroup during registration, you can set this from Users->Manage->Options->New User Registration Group.

To do this you can assign a condition always met to add user to some usergroup.
For example fields like firstname,username,email are always required, so you can create condition like:
Condition on username: if value is not equal to custom value (leave empty custom value) then add to usergroup A
This condition is always met because username is always not equal to a empty value