I use this hack
https://vborg.vbsupport.ru/showthread.php?t=171951
Sends invites from within your forums and you can customise the email too. Great thing about this is that once you've sent to one email you won't be able to repeat it to that email again, so at least no one can accuse you of spamming
I also have a buton like this
which links back to the members cpanel and their invite a friend screen See the attached screenshot
Good Luck