A component can be part of at most one Component Group (Hidden or any Group). If you want to have same component two groups, it is not possible directly but here is my suggestion.
Duplicate the existing component
Delete all the resources under copied component
Make original component as parent(sling:resourceSuperType)
Change the Component Group
I suggest to do this only if it is highly required, because this way it gives an impression like same component is under two groups but they are not actually one component. We are trying to achieve 100% inheritance here.
Note: sling:resourceType for these components will not be same once authored on a page