100. When Others are Saying Shema

17:9 Let's say that circumstances forced a person to stop in the middle of saying Shema - for example, he had to go to the bathroom. If the time he waited was long enough to say all of Shema, when he resumes, he must start again at the beginning.

17:10 If a person who already said Shema enters a shul and finds the congregation saying it, he should say it with them so he will not appear to be someone who does not want to accept the yoke of Heaven upon himself. Optimally, one should recite the entire Shema and not just the first verse. The reward for reciting Shema in such circumstances is like that of learning Torah. This is also the case if one is saying pesukei d'zimrah or is in other sections of prayer where he might interrupt. During pesukei d’zimrah, one might say only the first verse plus “Baruch Sheim…” -see Mishnah Brurah 65:10.) If a person who has not yet davened enters shul and they're saying Shema, he should say it with them but with the intention that it should not fulfill his obligation to do so. In this way, he may later recite it with the blessings when he davens. One should also recite other prayers with the congregation, such as Ashrei and Aleinu, so that he does not separate himself from the group.