Many years ago, I took a class about how to get started in wedding photography. The number one point of the entire class, was that most people fail because they don't charge enough, and that the amount of experience you have, shouldn't factor into your price.
If you are good enough, then you should charge a good rate for it. If you aren't good enough yet, then you have no business shooting a wedding (on your own).
There are several ways to build a portfolio without taking on the responsibility of being the primary photographer for someone's wedding.
This is what my brain telling me....IF they like the images I can sell them the prints
Just playing devil's advocate here, but what if they
don't like the images? Would you just shrug your shoulders and say "better luck next time...you get what you pay for" ?
That's not a good way to get started in weddings.
Also, if you shoot the wedding for free, but only hope to make money selling prints...then you are telling them that your time is worth nothing...and that will reflect in how they view you and treat you.
As an example:
Imagine it's the wedding day and after the ceremony you are trying to take photos of the B&G, maybe with their family or the wedding party etc. But all around you are the wedding guests, and most of them are also trying to take photos of the B&G. Most will be using their phones, but some will have cameras, one or two may likely have better cameras than you. The B&G will be standing there, with 20 cameras pointed at them, and they won't know where to look or who to pose for. They see a bunch of friends and family, people they know and love...and then there is you, someone who they didn't pay to be there. It's not a good scenario to be in, and it makes it hard to get the good photos that you want to create for them.
And when you want to take them away from the crowd of guests, to get some photos...how much of a priority are you going to be? It's not like they're loosing any money if they don't give you much time.
Sure, they would want nice photos (who wouldn't?)....but if they are the type of people who 'hired' a free wedding photographer, they probably don't care as much about the photos as they should. And again, this is not a good scenario for you. It can be pretty hard to get great photos of people who don't care enough about helping you to get great photos.
But if they have paid you, especially if it's a significant amount, they will be much more likely to pay attention to you. From a selfish (but understandable) standpoint, they would want to get their money's worth, so the would want to work with you and hopefully have you as a higher priority than the guests with cameras. But from a better standpoint, if they are the type of people who are willing to pay for good photography, and that alone makes for better photos.
*edit* Had to run off before finishing my post
So my point with the example, is that it's not a good idea to charge nothing, especially with the attitude of 'maybe they will buy prints if they like them'. You need to be confident and know that your time is valuable and you shouldn't just give it away. People will respect you more for that...and clients who respect you, will often make for better photos, which makes you happy and them happy.
Now, with all that being said...we all have to start somewhere. And I'd venture that most wedding photographers have stories of similar situations from their early days. I would suggest trying to find a way to be involved but you may not want to assume the responsibility of being the THE wedding photographer.
Most of my first wedding experience, was for family members who asked my to shoot their weddings, but I told them to hire a pro for the most important things like the ceremony and formals, but that I would shoot the rest. Everything from getting ready to the last dance at the end of the night.