Learn Arabic (Modern Standard): سنقرأ الخطة معا.

The letter سـ (sa-) attached to the very beginning is the future prefix. You just add it directly to a present tense verb to make it future tense. Without it, نقرأ (naqra'u) just means "we read" or "we are reading".

Could we use سوف instead of سـ here?

Yes, absolutely! You could write سوف نقرأ الخطة معا. Both are used for the future tense. The main difference is that سـ is a prefix attached directly to the verb (often for the near future), while سوف is a separate word written before the verb.

Where is the word for "we" in this sentence?

It is built right into the verb. After the future prefix سـ, the letter نـ (noon) in سنقرأ is the present tense marker for "we". You do not need to add the separate pronoun نحن (we), though you could if you wanted to add extra emphasis.

I recognize مع as "with". Why does it have an alif at the end here (معا)?

Adding that alif with a double vowel sound (called tanween fatha) turns the preposition مع (with) into the adverb معا (ma'an), which means "together". You will see this ending a lot in Arabic to create adverbs!