We have a custom workflow, we initiate workflow from siteadmin properties and assign workflow initiators  and approvers  for specific path. If we assigin multiple approver groups how can we get the multiple groups in dynamic participant step?
You can create a custom Dynamic Participant workflow step by implementing ParticipantStepChooser. When creating a Dynamic Participant Step, you can use application logic to determine whom to assign the workflow item. In your logic, you can get group information too. All of this can be coded.
Dynamic participant step is the right way to choose a participant from the code, but it won't allow you to choose multiple participants through same step. I guess Kishore's requirement is to select multiple groups through same participant step.