Blessing Shehechiyanu:[1]

One who has not said the blessing of Shehechiyanu by candle lighting is to say the blessing prior to the evening prayer.[2] Ideally, it is best for the congregation to listen to the chazzans blessing and fulfill the obligation through him, being that it is more beloved to a King when His commands are fulfilled in public unity.[3] Nevertheless, the custom is for every individual to say the blessing, being that the Chazan does not have anyone else in mind when saying his blessing.[4]

Finishing the blessing prior to the Chazan:[5] When saying the blessing of Shehechiyanu, one must beware to finish the blessing prior to the Chazan, in order to answer Amen for his blessing.[6] [It is implied that one should say the blessing together with the Chazan and simply finish before he does, rather than say it afterwards.] 

_____________________________

[1] Admur 619/7

^[2] The reason it is not said over wine: Although on all holidays the blessing of Shehechiyanu is said over wine by men, nevertheless, on Yom Kippur, being that its forbidden to drink, this cannot be done, even if one were to say it over wine prior to the start of Yom Kippur, being that once the blessing has been said, its considered that he accepted the holiday and thus all the laws of Yom Kippur now apply to him. As well, giving a child to drink from the wine is also not done, being that children may only be given to eat on Yom Kippur for their own sake, in order not to accustom them to eating on Yom Kippur even when they get older. [Admur ibid; See M”A chapter 556 regarding Tishe Beav]

[3] Admur 619/8; Vetzaruch Iyun as to how this does not contradict the ruling of Admur 213/6 which states that by Mitzvos fulfilled individually [such as Lulav, and Hallel, Vetzaruch Iyun as to the definition of Birchas Shehechiyanu] one can choose whether to say it himself or be Yotzei with the Chazan, and there is no advantage either way. [See Hearos Ubiurim Ohalei Torah 805/72 who asks this question and suggests that Shehechiyanu is considered a collective Mitzvah, while the extension of Hallel and Birchas Lulav was never intended to be going on the beginning of this Halacha but rather on the second part that the congregation should finish before the Chazan. However see M”A 619/3 who does not learn this way]

[4] Admur 619/8

[5] Admur 619/8

^[6] The same applies for the blessings said in Shul over Lulav and Halel. [ibid; Back then the custom was for the chazzan to say the blessing over the Lulav aloud right before Hallel. This is no longer practiced today as explained in laws of Sukkos]

Why isn’t the blessing just said after the chazzan finishes his blessing? Perhaps it is more public and thus more beautified when said together with the Chazan.