The reason for this is so that it shouldn't seem to any onlookers (who may not know that he already davened and/or recited Krias Shema) as if he doesn't want to accept Hashem's reign upon himself, as the rest of the congregation is doing.

 

According to some Poskim (the Gaon of Vilna, the Kitzur Shulchan Aruch and others) in the above scenario the entire Krias Shema must be said with them not just the first Posuk. If possible, it is ideal to be stringent in this matter.

 

2) If the individual did not yet recite Krias Shema, and happens upon the congregation saying it, although he must say along with them , he should [specifically] have in mind not to satisfy his obligation with this recital, rather it should be like "reading/learning Torah".

 

The reason for this is so he can satisfy his obligation later on properly (i.e. during davening, along with the Birchos Krias Shema)

 

One who did not yet recite Birchas HaTorah and happens upon a congregation saying Krias Shema may only say the first Posuk with them , as saying the rest would constitute learning Torah before Birchas Hatorah, which is strictly forbidden.

 

In fact, if one who did not say Birchas HaTorah is outside of the Shul, and he realizes that the congregation is about to say Krias Shema, it is better for him not to go inside, as to avoid saying even the first Posuk of Shema without Birchos HaTorah.(Sefer Orach Ne'eman)

 

Reader's Question:

 

I recently davened at a minyan where the shliach tzibur checked the tzitzit on his tallis, thereby delaying the start of mincha a few monemts past the scheduled time. Was this proper? 

  
 
My Answer:  

 

Although it is important for the Public Tallis to be checked as often as possible, it should not be done at the expense of the waiting Tzibbur(as we can in certain cases rely on the Chazaka that if it was kosher yesterday and the day before, it's probably still kosher today).

 

Rather, the Shatz should try and do it a moment or  two before the scheduled time.

Our holy sages, the Chazal tell us  

  תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן עולם הבא, שנאמר הליכות עולם לו, אל תקרי הליכות אלא הלכות
- one who studies [at least two] Halachos daily is assured a portion in Olam Haba -  the world to come. 
(:מגילה כ"ח )

 

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