"Everyone has arrived, where are you now?"
现在 is the time word "now". Grammatically, it works like any point in time: 今天，去年，这个周末，etc. (It is placed before or after the subject.) 正在 modifies the verb. The mening is like adding "-ing" to a verb in English (although it is not used in as many circumstances in Chinese as in English). It shows that the verb is IN PROGRESS RIGHT NOW. 在 alone can also be used for verbs in progress, but 正在 puts even more emphasis on the "happening right now" aspect. Both 在 and 正在 are placed directly before the verb they modify.
都 is an adverb, so it can't be the subject of the sentence. 大家 is a pronoun. 大家都+verb just emphasizes the all-inclusive nature. (It does not technically change the meaning of the sentence from just 大家.)
大家都 is commonly used in Chinese. (It does not sound wordy as it would in English.)