how can invite somebody to follow? and how can this work with nondocument related emails? in the pre-opportunity status?

If you want to invite people in group then form my group-> join group, open the group in which u want to invite. then on the right side there is button "invite people". from here you can invite other people.

and if you want to invite anyone on some document then open that document and click on the right side there is button "invite people".

And what about Non Document related =private Discussion? what not have model , and res_id? how can "forward"?

there is something i don't understand. when a partner sends a message, the message is sent to a company user (with his adress) and not to the group he belongs to. So how is this message forwarded to the groups the receiver belongs to ?