Invitation for specific player
As a player who's been invited, rMail is my preferred method. You can send a link to the game, so it's easy for the player to find, and they have the option to join or not join as they see fit. If it's your first contact with that player as a GM, an initial message, along the lines of, "I think you'd be a good fit in my game...if you're interested, I'll send you a link," is a good way to start. You're not advertising your game, you're not spamming people, and if they have no interest, they can bow out gracefully at that point. I've actually been approached that way two or three times...people who'd seen some of my posts here in Community Chat and felt like my approach to gaming was a good fit with theirs.
If it's someone you already know as a GM, you could either PM them in game, or rMail them (I'd prefer rMail so I've got less clutter that's not about the game.) If they're a fellow player, I'd PM them in the game you play together, just because that's the easiest way for them to identify who you are (I've been sent rMail from other players and it's occasionally a little tricky to place where I know them from.)
But however you message them, start with an "Are you interested?" inquiry before sending them a link or the name of the game for them to search. To me, that's a more polite invitation, rather than you pushing the game at them.