I've got a secret for you. I'm trying to take over the world, one donut shop at a time.
There is nothing wrong with a cadre group joining some larger one and endeavoring to pitch their own bloomin philosophy. My guess is that your argument is not with colonization or 'entrism,' but with vanguardism. You're entitled to your point of view, but it's obnoxious and self-defeating to raise it with language that is indistinguishable from right-wing 'outside alien agitator trouble-maker' stuff. This sort of language can help Right more than serve any left purpose.
Entrism is obnoxious when it is not done in an above- board manner. My first taste of this was in SDS, in a PL dominated chapter. The domination was not so bothersome as the fact that a PL/non-SDS person would be outside the meeting, listening at the keyhole, as it were. Nobody would have cared if he had come in and participated. It was just stupid, but it made me feel like a guinea pig of some kind. Bad vibes.
Entrism is annoying when it is manipulative, when the decision-making process is bent to favor the position of the colonizers, when its practitioners are unprincipled, and obviously when the politics of the entrants just stink.
Otherwise there is nothing wrong with it, and something wrong with the ways in which it is sometimes denounced.
mbs