The sentences need a little work.
You mixed your tenses in that sentence. Try: They are getting ready for their wedding party, which is later this evening.1-They are preparing their wedding party later this evening.(Their party which will take place later this evening)
They are getting ready for their wedding party, which is at five o'clock.2-They are preparing their wedding party at five o'clock.(The party which will take place at five).
He is getting ready for his wedding party, which will be at the hotel.3-He is getting prepared for his wedding party at the hotel. (The party will take place at the hotel, but he isn't at the hotel right now.)