Now, all of you will come in groups of five near the table, turn by turn and look carefully at the light falling on the ball.

here "in" after verb "come" is an adverb or apreposition?

what is meaning of turn by turn? Does it mean one group after another?

'In' is a preposition, and the phrase 'in groups of five' is an adverbial of manner. 'Turn by turn' seems odd to me; I would have expected 'in turn(s)' perhaps; but it does mean 'group by group' in this sentence.
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Thanks a lot for such a wonderful explanation.

I have one more question.

I sometimes notice that adverbs like "now", "today" etc are seperated by comma, and somtimes do not?

1.Now teacher said ,"--------------.".

2.Now, We will rotate the glove.

What are the rules about the comma in this case.

First, remember that there are two 'now's, Hanuman:

(1) adverb of time ('at this moment') -- 'I want my lunch now'.
(2) sentence adverb ('next topic', 'by the way') -- 'now, I see that you are unsatisfied with my explanation'.

The second one takes a comma quite commonly-- but not necessarily-- because it modifies the entire sentence. It is also common for other sentence adverbs, particularly if they begin the sentence, to be set off by a comma:

'Today, we are going to examine the results of our experimental cross-breeding'.

Often, the comma is an option available to the writer; if the sentence is short and simple, it may not be needed:

'Today I want to start work early.'