How to add vertices into specific vertex groups

Question

I have an xml file which contains a list of objects in space ("sectors") each sector has a list of more objects ("seats") which can be the same model or not. For each "sector" I create an object and for each seat in that sector I create a face (made up of 3 verts) from which a particle system will instantiate a mesh using its normal. Till here no issue, my problem arrives when I try to put each face (seat) into a vertex group which i create and which will contain all the faces "seats" representing each seat model.

I can`t manage to assign a list of vertices into a vertex group, I'been struggling with the vertex_group.add, it always returns me this error:

group.add(selected_verts, 1.0, 'ADD' )
AttributeError: 'bpy_prop_collection' object has no attribute 'add'

location: <unknown location>:-1

Here is the code, I hope it is more self explicatory than my description:

import bpy
import os
import bmesh
import math
import mathutils


from xml.etree import ElementTree
from bpy.props import StringProperty, BoolProperty
from bpy_extras.io_utils import ImportHelper
from bpy.types import Operator



def createMonkeyFromData(tree):

    for estadio in tree:
        #Create Venue parent
        O = bpy.data.objects.new('venue', None)
        bpy.context.scene.objects.link(O)

        for Sector in estadio.findall('Sector'):
            #Create Sector
            mesh = bpy.data.meshes.new('mesh')
            sector = bpy.data.objects.new(Sector.get('id'), mesh)
            bpy.context.scene.objects.link(sector)
            bpy.context.scene.objects.active = sector
            bm = bmesh.new()

            total_seats = 0

            #Create Tiers
            tier = str(Sector.get('tier'))
            if bpy.data.objects.get(tier) is None:
                tier_O = bpy.data.objects.new(tier, None)
                bpy.context.scene.objects.link(tier_O)
                tier_O.parent = O
                # Parent sector to venue empty
                sector.parent = tier_O

            for Seat in Sector:

                seat_type = str(Seat.get('prefab'))

                if 'px' in Seat.keys():
                    # Get Position Data
                    posX, posY, posZ = float(Seat.get('px')), float(Seat.get('py')), float(Seat.get('pz'))
                    rotX, rotY, rotZ = float(Seat.get('rx')), float(Seat.get('ry')), float(Seat.get('rz'))

                    #Total number of seats for each sector
                    total_seats = total_seats + 1
                    #Create vertex and face for each position and orient it
                    v1 = bm.verts.new((posX-0.01, posZ, posY-0.01))
                    v2 = bm.verts.new((posX+0.01, posZ, posY-0.01))
                    v3 = bm.verts.new((posX, posZ, posY+0.01))
                    f1 = bm.faces.new([v1,v2,v3])
                    vert_list = [v1,v2,v3]
                    mat_rot = mathutils.Matrix.Rotation(math.radians(-rotY), 3, 'Z')
                    bmesh.ops.rotate(bm, cent= (posX, posZ, posY), matrix = mat_rot, verts= vert_list)

                    selected_verts = [v.index for v in vert_list if v.select]


                    bpy.ops.object.mode_set(mode='OBJECT')


                    #Detect seat and create vertex group
                    if bpy.context.object.vertex_groups.get(seat_type) is None:
                        group = bpy.context.object.vertex_groups
                        group.new(name=seat_type)
                        group.add(selected_verts, 1.0, 'ADD' )




            bm.to_mesh(mesh)
            bm.free()

The vertex groups are created with no problems but vertices wont get assign to its corresponding group, any idea on what I'm missing?

EDIT

As @sambler suggested I've tried selecting the vertices inside a loop but still not working, which is very strange because the script @sambler shared works fine. Right now the code has been updated like this:

                #Detect seat and create vertex group
                for v in f1.verts:
                  if bpy.context.object.vertex_groups.get(seat_type) is None:
                    group = bpy.context.object.vertex_groups.new(name=seat_type)
                    f1.select = True
                    group.add([v.index], 1.0, 'ADD' )

The group.add is not assigning the vertices to its vertex_group

Answer 1

Your group variable is the list of all groups, it needs to be a single group. Get the new group returned by groups.new() and use that.

group = bpy.context.object.vertex_groups.new(name=seat_type)

Then as you are using bmesh, the way to assign weights is different. We need to get the bmesh deform layer and use that with the group index to assign a weight to a bmesh vertex.

import bpy
import bmesh

obj = bpy.context.object

bm = bmesh.new()
bm.from_mesh(obj.data)
dl = bm.verts.layers.deform.verify()

for x in range(4):
    for y in range(4):
        group = obj.vertex_groups.new(name=str(x)+'_'+str(y))
        v = bm.verts.new((x,y,0))
        v[dl][group.index] = 1.0

bm.to_mesh(obj.data)
bm.free()