Traitors and mind control.

Cybermen tend to invade by stealth, often employing human agents to do their dirty work whilst they prepare to strike in earnest. Tobias Vaughn in The Invasion and Kellner in Revenge of The Cybermen are probably the best examples of this.

Alternatively, some Cyberman seem to have the power to control minds, creating willing slaves to do their bidding. If your mate starts talking in a flat monotone and staring blankly ahead, they’ve probably been got. Either that, or they’ve watched The Wheel in Space too many times.