One of the drawbacks of not being the official photographer is that you don’t have the attention of the happy couple at some key points.
This can produce some near-miss shots that are nicely composed, but without the essential eye contact.
The bride and groom won’t appreciate you calling for them to look at you while the pro is supposed to be getting the shots that they are paying for, so bide your time.
Most pros understand that the family want to get a few shots of each pose and you need to be ready to seize your moment.
If you fit a longish lens you can also snap some intimate moments between the bride and groom when they look at each other rather than the camera.