I have a friend who became a Mormon. Didn't think much of if until his wedding. Atop of the church spire was a golden figure blowing a trumpet.
Thinking it was a saint i asked someone who it was, "Joseph Smith" they proclaimed, he is their prophet.
We entered the church as the ceremony was beginning but every thing stopped and we were asked to leave the church.
Apparently only Mormons are allowed in the church. After that I learnt all about their beliefs and was really surprised.
The whole story of how the angel appeared to Smith and directed him to these golden tablets is hard to believe.
It gets even harder to believe given the fact that he didn't show them to anyone and how he would put his head in a basket to translate them.
The language which it was written in was put to experts to figure out. They concluded that the text was of no known language on earth.
Aside from that their doctrine includes many other things that are unique to only them
Actual member of the Church of Jesus Christ of Latter-Day Saint (nick named "Mormon") here going to clarify things.
Your friend was not getting married at his local church building where he attends church each week, partakes of the Lord's Supper, has Sunday School etc. You (or anyone else) are indeed welcome to come, ask whatever, and celebrate Christ with us.
Now zooming to your friend's marriage. There's two components of a wedding: A) the religious and B) the secular/civic side. I'll talk about both.
A) He was simply getting civically married like you would at a courthouse. Rather, there's strong belief in the religious aspect marriage: that this is an extremely sacred ceremony wherein he, his bride, and God are making promises to each other to be unified together not just for this mortal life but forever. It's viewed as being very sacred religious thing, and hence only a very few guests and just simple religious rite putting God first.
B) Secular/Civic: besides the religious rite, your friend of course is welcome to have any secular traditions he would like separately and whomever comes. Stuff like a million flowers, bridesmaids, aisle walking, ring exchange, reception, dancing, etc. Almost all LDS Christian couples will have these as well, and invite a ridiculous amount of people.
Anyway, any other questions let me know!