"For certain people" - The group of heretics Jude was addressing were limited in number but broad in their effect, so much so that they themselves and their ideas were almost inseparable at this time. Certainly, they didn't really creep in unnoticed. Their ideas did.

"crept in unnoticed" - Heresy doesn't enter the church with fanfare and announcements. Nor does it come all at once. It arrives quietly, and takes it's time, making sure not to make a big stir at the start. It convinces people, one at a time, not of major doctrinal lies but of small adjustments to "practice." The real effect is not noticed until months or years later.

"who long ago were designated" - In spite of the sneakiness of this group and it's ideas, God was not surprised. In fact, he was behind it, in a sense. He designated, even designed, these certain people to introduce this certain falsehood. My best guess is that he probably did that in order to solidify the truth for the church.

Jude 1:4