What is the proper doctrine/mysql definition to allow users to one conversation?

  database, doctrine-orm, doctrine-query, mysql, symfony

Currently I have a message system (for example, like eBay between two different users, one conversation, two members). Now I want to allow members from the conversation to invite other members to the active conversation (new members will be able to read previous messages). What should be the proper way to define the entity in doctrine or Mysql? One entity called "Message" (the firt message), linked to another entity called "Chat" (using OneToMany) and to connect messages and users to use a ManyToMany relationship?

I am stuck at that point as I do not find any information about it, only about regular message system between two users, even between three but if I want to allow unlimited users things get more difficult. Thanks

Source: Symfony Questions