Here is the possible code to import the users with groups.

import xmlrpclib
url = "http://localhost:8069"  
db = "testDatabase" 
username = 'admin'
password = 'admin'
common = xmlrpclib.ServerProxy('{}/xmlrpc/2/common'.format(url))
uid = common.authenticate(db, username, password, {})
models = xmlrpclib.ServerProxy('{}/xmlrpc/2/object'.format(url))
user_id = models.execute_kw(db, uid, password, 'res.users', 'create', [{'name':"userAPI", 'login':'userapi@gmail.com','company_ids':[1], 'company_id':1, 'new_password':'1234567890'}])
sale_module_categ_id = models.execute_kw(db, uid, password, 'ir.module.category', 'search', [('name', '=', 'sales')])
#SEARCHES AND LINKING CUSTOMER TO THE GROUP SALES MANAGER
if sale_module_categ_id:
	sale_manager_group_id = models.execute_kw(db, uid, password, 'res.group', 'search', [('name', '=', 'sales '), ('category_id', '=', sale_module_categ_id[0])])
	if sale_manager_group_id:
		models.execute_kw(db, uid, password, 'res.group', 'write', sale_manager_group_id, {'users': [(4, user_id)]})
#SEARCHES AND LINKING CUSTOMER TO THE GROUP WAREHOUSE MANAGER
warehouse_module_categ_id = models.execute_kw(db, uid, password, 'ir.module.category', 'search', [('name', '=', 'sales')])
if warehouse_module_categ_id:
	warehouse_manager_group_id = models.execute_kw(db, uid, password, 'res.group', 'search', [('name', '=', 'sales '), ('category_id', '=', warehouse_module_categ_id[0])])
	if warehouse_manager_group_id:
		models.execute_kw(db, uid, password, 'res.group', 'write', warehouse_manager_group_id, {'users': [(4, user_id)]})

Hope this helps.

Thanks.