To get married you, your partner and witnesses will need a minimum rank of Mobster. If you are male you can propose to the woman you love through this option.
If you are a female, you can set whether you would like to receive wedding proposals or not. To be able to marry you both need a apartment/house/villa in the city where the marriage will take place.
Costs of the wedding vary from $250.000 to $2.500.000. You need 2 witnesses for the ceremony, they can give you wedding gifts, all the players has to be in the town where the wedding takes plays.
You cannot marry the same gender as yourself. But you are allowed to marry one on the same IP as yourself, and have witnesses which are on the same IP.
If your husband/wife dies and you find a nice shoulder to cry on, you can marry him/her right away. But if you married the wrong, you can divorce the person, it will cost you 2 million.

Keep in mind! It is only possible to attend a wedding if not invoilved in shooting in last 24h.