serializers.py

class GroupSerializer(serializers.ModelSerializer):

    class Meta:
        model = Group
        fields = ('id', 'name')


class UserSerializer(serializers.ModelSerializer):

    all_groups = GroupSerializer(source='groups', many=True)

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