serializers.py

class UserSerializer(serializers.ModelSerializer):

    groups = serializers.SerializerMethodField()

    def get_groups(self, obj):
        groups = obj.groups.all()
        if not groups:
            return None
        return GroupSerializer(groups, many=True).data

    class Meta:
        model = User
        fields = ('email', 'first_name', 'last_name', 'groups')