May one pray before sunrise?
Based on the Gemara’s ruling, the Shulhan Aruch (O.H. 89:1) states that the ideal time to pray in the morning is at sunrise. Practically speaking, this means that the earlier parts of Shaharit should be timed and recited such that one begins the Amida right at sunrise. The Shulhan Aruch continues and says that if one prays as early as when the eastern horizon begins to light up, one still fulfils one’s obligation ex post facto.
The Shulhan Aruch (ibid:8) mentions cases when there are extenuating circumstances, such is if one must travel very early. In such a case, one may pray as of Amud HaShahar (dawn). Another contemporary example is if one lives in a part of the world in which sunrise is relatively late and one must pray earlier at dawn in order to arrive at work on time.
There is a discussion regarding one who is not faced with pressing circumstances but can only find a Minyan that prays at dawn. On one hand, the Gemara says that the optimal time to fulfil the Mitzvah of the morning prayer is at sunrise. On the other hand, there is great benefit to praying in a quorum. Rabbi Yitzhak Blazer (Pri Yitzhak, § 2) says that in such a case one should pray at dawn with a Minyan. Rabbi Ben Zion Abba Shaul (Or Lezion, vol. II, ch. 7, § 4) cites the Pri Hadash, who unlike the Shulhan Aruch, says that one may optimally pray when the eastern horizon is lit up, which is approximately 50 minutes before sunrise. However, he says that if the Minyan is praying at the beginning of dawn, which ranges between 72 and 90 minutes before sunrise depending on the opinion, then it is preferable to pray alone at sunrise.
On the other hand, Rabbi Ovadia Yosef (Yabia Omer, vol. IX, 9:108,45), quoting the Rambam (Teshuvot HaRambam, § 26), follows the Shulhan Aruch more literally and says that any time before sunrise is not optimal. Therefore, in such a situation, he rules it would be preferable for the individual to pray alone at sunrise and not with the Minyan earlier. Due to the importance the Gemara ascribes to the time of sunrise, it appears that this is the proper practice.
Summary: One should pray Shaharit at sunrise. In extenuating circumstances, one may pray as early as Alot HaShahar. If one is not pressed for time but can only find a Minyan before sunrise, it is preferable to pray alone at sunrise.