Save chat conversations via REST API

Is there any way to save the chat conversations?

I am using an architecture as below.

Client : ASP.NET SignalR JavaScript Library v2.0.0
Server side : Microsoft.AspNet.SignalR.Hubs(To communicate with client) + Matrix Library(To communicate with
open fire using XMPP Client)
Database : SqlServer

I want to know if openfire has any Service/API to save chat conversation.

I am using openfire 4.2.1. I have downloaded the REST API plugin. I could create/update/delete/search user and group. I want to know how to add users to group.

There is an API to add users to group.

/users/{username}/groups

But to which group the user will be added.

Open fire shows the details about the client sessions.

Also please let me know how group chat can be achieved. I checked the below link,

But, I could not get more details about group chat.

Thanks & Regards,
Anoop H.N.