I'm going to take a different angle here.  I'll admit its frustrating when a well-meaning guest pops up in front of me with their #iPhone blocking my shot and most definitely coming back with a blurred mess.  So rather than fight it here's a tip that seems obvious but needs repeating.

Light.  You MUST have light when using a phone for photographs.  If you are at an outdoor, daytime wedding unless its overcast you will have high contrast.  Get up close.  No not during the ceremony silly.  But after.  You need light on their faces.  And them close to the camera.  Or at the very least lots of light on them.  Even light.  No harsh shadows or blown out whites.  If its dark or night time honestly give up, unless you have some great light close by you can use.  The flash on these phones still is rather sketchy.  Plus the color is usually hideous.    

There are lots of apps for holding focus while adjusting exposure.  I just tap the screen in the darkest area closest to the same plane as the subject. If you are at least 10' away you should have no problem.  If the subject is up close to you, tap on their face.  Its the most important part for proper exposure and focus.  

Also another little trick I use is tilting the phone, finding the best exposure. Tapping the screen and holding while tilting the phone back into the correct plane.  If the sky is bright relative to the foreground, try tilting the phone up or down a little.  Might work if tapping doesn't.  

The portraits above are from a first look we did on the levee one very cold December morning.  I made sure I would have even light on their faces and bodies so I chose a specific time in the morning.  The app I used was the #hipstamatic.  So I have no control over focus.  But I know with the particular film and lens combo used the subjects needed to be centered.  Its a complicated and processing intensive app.  I think I probably used PhotoForge which isn't available any longer.  Then lastly Lightroom and Photoshop. 

Have fun, point and take some awesome shots but do the wedding couple a favor, put your phone down till after the ceremony. Unless the light is just perfect you won't get that great of a shot and miss the important moments so important to them.