Bring into the conversation, quietly

• Secretly include on an email
• Secretly loop in
• Include secretly in an email, say
• Abbr. in an email field
• Secretly add to the email thread
• Loop in secretly
• Include secretly, as in an email
• Secretly keep in the email loop, briefly
• Include surreptitiously, in a way

Wiktionary Word definitions in Wiktionary
init. 1 body-centered cubic 2 blind carbon copy vb. to send something by blind carbon copy