In addition to it being annoying, it will probably have the reverse effect of what you're after, i.e. people won't want to come back. If your channel is a good one, you shouldn't need to do that - people will keep coming back regardless of whether you invite them or not. And bear in mind that if someone has parted your channel, it could be for a variety of reasons, and constantly inviting them back might just seem...well, desperate.