Torah reading was established for the community as a whole, and does not apply to each and every individual (Ramban Megillah 5a). Therefore, a person who had to leave in the middle of the Torah reading and missed part of it, need not find another minyan in which to make up what he missed, because the important thing is that the congregation fulfilled the mitzvah of Torah reading.
Someone who has the following two options: to pray in a minyan and leave before Torah reading, or to hear the Torah reading in a minyan, but pray individually – it is preferable that he pray in a minyan because an individual is commanded to pray in a minyan, whereas the mitzvah to read the Torah is a communal commandment and does not pertain to individuals (see Minchat Yitzchak 7:6 and Piskei Teshuvot 135:2). Likewise, even if a person who had to pray individually later discovers a minyan in which the Torah was not yet read, he is not obligated to go join them and hear the Torah being read there (Yalkut Yosef, part 3, 135:7).
If a person arrives late to synagogue, and when he is reciting Pesukei d’Zimrah or Birkot Keriat Shema, the congregation starts to read the Torah, if he will have a chance afterwards to hear the Torah reading, he should continue to pray. However, if another opportunity to hear the Torah reading will not arise later, l’chatchilah it is best that he stop praying and listen to the Torah being read (Leket Yosher p. 18; Yabia Omer 7:9).
If six people who prayed individually, but did not yet hear the Torah reading, assemble in the morning, another four people may join them to read the Torah (Bei’ur Halachah 143:1).
Even if they only convened in the afternoon, according to many Acharonim, they may make up the Torah reading at Minchah (Mishnah Berurah 135:1). However, some disagree and maintain that the Torah may not be read in the afternoon. Nonetheless, in practice, those who wish to make up the Torah reading in the afternoon are permitted to do so, and that is how many prominent Jewish rabbis practiced (Shut Yehudah Ya’aleh, Orach Chaim 51). Therefore, those who did not have a Torah scroll for Shacharit, such as a minyan of soldiers or travelers, upon arriving at a place in the afternoon with a Torah scroll, may read the Torah and make up what they missed (see Yabia Omer 4:17; Piskei Teshuvot 135, note 24).
