add docs for fix microsoft#2643

website/docs/topics/groupchat/using_custom_models.md

@@ -0,0 +1,79 @@
+# Using Custom Models
+
+When using `GroupChatManager` we need to pass a `GroupChat` object in the constructor, a dataclass responsible for 
+gathering agents, preparing messages from prompt templates and selecting speakers 
+(eventually using `speaker_selection_method` as described [here](customized_speaker_selection.mdx)).
+
+To do so GroupChat internally initializes two instances of ConversableAgent.
+In order to control the model clients used by the agents instantiated within the GroupChat, which already receives the 
+`llm_config` passed to GroupChatManager, the optional `model_client_cls` attribute can be set.
+
+
+## Example
+First we need to define an `llm_config` and define some agents that will partake in the group chat:
+```python
+from autogen import GroupChat, ConversableAgent, GroupChatManager, UserProxyAgent
+from somewhere import MyModelClient
+
+
+# Define the custom model configuration
+llm_config = {
+    "config_list": [
+        {
+            "model": "gpt-3.5-turbo",
+            "model_client_cls": "MyModelClient"
+        }
+    ]
+}
+
+# Initialize the agents with the custom model
+agent1 = ConversableAgent(
+    name="Agent 1",
+    llm_config=llm_config
+)
+agent1.register_model_client(model_client_cls=MyModelClient)
+
+agent2 = ConversableAgent(
+    name="Agent 2",
+    llm_config=llm_config
+)
+agent2.register_model_client(model_client_cls=MyModelClient)
+
+agent3 = ConversableAgent(
+    name="Agent 2",
+    llm_config=llm_config
+)
+agent3.register_model_client(model_client_cls=MyModelClient)
+
+user_proxy = UserProxyAgent(name="user", llm_config=llm_config, code_execution_config={"use_docker": False})
+user_proxy.register_model_client(MyModelClient)
+```
+
+Note that the agents definition illustrated here is minimal and might not suit your needs. The only aim is to show a 
+basic setup for a group chat scenario.
+
+We then create a `GroupChat` and, if we want the underlying agents used by GroupChat to use our
+ custom client, we will pass it in the `model_client_cls` attribute. 
+
+Finally we create an instance of `GroupChatManager` and pass the config to it. This same config will be forwarded to 
+the GroupChat, that (if needed) will automatically handle registration of custom models only.
+
+```python
+# Create a GroupChat instance and add the agents
+group_chat = GroupChat(agents=[agent1, agent2, agent3], messages=[], model_client_cls=MyModelClient)
+
+# Create the GroupChatManager with the GroupChat, UserProxy, and model configuration
+chat_manager = GroupChatManager(groupchat=group_chat, llm_config=llm_config)
+chat_manager.register_model_client(model_client_cls=MyModelClient)
+
+# Initiate the chat using the UserProxy
+user_proxy.initiate_chat(chat_manager, initial_message="Suggest me the most trending papers in microbiology that you think might interest me")
+
+```
+
+This attribute can either be a class or a list of classes which adheres to the `ModelClient` protocol (see 
+[this link](../non-openai-models/about-using-nonopenai-models.md) for more info about defining a custom model client 
+class).
+
+Note that it is not necessary to define a `model_client_cls` when working with Azure OpenAI, OpenAI or other non-custom 
+models natively supported by the library.