What camera and what lens are you using? Do you have a flash on with AF-assist? Always use a cross AF sensor whenever possible with as large an aperture as possible. AF-assist is/can be invaluable, but use the flash af-assist, not the one on camera.

You can still use it the flash to lit up the environment as it can get really dark, but the spotlight if they handle it correctly, it should light up the subject well; I do experience those spotlight didn't follow the couple properly and sometimes only 1 person is in the spotlight.

Cross AF sensor, means the main AF point; Just use the middle AF point, and set to AF-C (continuous focus). For march in, usually they should walk slowly, you should be able to get more usable shoot than those walking pace.