I would agree with instinct71. It's used as follows:
- say an email is sent to a list of recipients, but someone was omitted or the topic needs to include other recipient(s). The new recipient(s) are added to the To: or CC: fields and their names are also added to the body of the email with a ++ or + , just to inform everyone on the current distribution that others have been added to the discussion.
"++" came from C programming and non-programmers decided to reduce that to a simple "+"
There is slight difference I have observed in usage of + and ++.
'+' Usually follows a name, to be informed or to action on.
'++' I disagree that this is a programming jargon despite I being a programmer. ++ usually does not follow a name. This is used when more than one person has been added to email. Sometime, It may be followed by names if possible. But mostly, it is to indicated that additional peoples have been added but need to specifically mention the names is not required or it is not possible to mention the names.
I have usually seen either "+SimpleMan" or simply "++". My guess is that this comes from computer programming jargon.
In programming "++" is a shorthand for "add 1 to a variable". For example: "a++" increases a's numerical value by 1. With emails, the variable under consideration is the list of email recipients. "+SimpleMan" (although not legal programming AFAIK) is specifically adding SimpleMan to the list of recipients.