Only rebbes throw this party? Or is it just that only they can afford it?
-1 IINM the Chusen and Kallah (to be), dance Mitva Tantz at the Forshpiel, in some places.
To clarify, there are several pre-wedding parties. (These are by rebbish only, and not all apply to every chassidus.) There's the furshpiel on motz"sh before, as explained above. There's something called an urime chassuna/urime seuda. This stems (and in some places still is) from when they actually invited all poor people of the town and gave them a hefty dinner. Today it is associated with (in addition to the seuda) giving tzedaka to many poor, or a bonus check to the kollel yungerleit from that kehilla etc. This party takes place 2-3 days before wedding, many times more than that, and even before shabbos aufruf, though. The night before the wedding (usually) there's a party called a chussen mul. This has two parts: 1, hakafos; where the chassan goes around the city/town/marketplace/shul/main living area (in a car) 7 times. Often he is accompanied by chassidim with torches singing etc. 2, is the actual seuda after which the chassan and kallah dance their mitzva tantz instead of during the chassuna.
Not in all places do they do these things, and in some places they only do some of it, while yet in others they combing two or more of these events. But a rebbish chassuna sure does involve a whole lot of partying!