It is not often, but once in a while I get recruited to join another guild. From my point of view these people just spam guild invites without taking into consideration whether or not that person is already in a guild.
The only reason to attempt to recruit someone who is already in a guild is if they are actively complaining about their current guild. If a player is in a guild don't ask them to join yours unless you know for a fact they are looking for a new guild.
Or if you make ads for your guild and someone PMs you that they want to join