Learn Arabic (Modern Standard): المقاولون حاضرون في الاجتماع. - The contractors are present at the meeting.

Both المقاولون (the contractors) and حاضرون (present) are sound masculine plurals in the nominative case. In an Arabic nominal sentence (a sentence without a verb, like this one), both the subject and the predicate take the nominative case, which is marked by the suffix ون for regular masculine plurals.

Why isn't there a word for 'are' in this sentence?

Arabic doesn't use a 'to be' verb (like 'is' or 'are') in the present tense. By simply placing the subject المقاولون (the contractors) and the predicate حاضرون (present) next to each other, the word 'are' is automatically understood.

Why doesn't حاضرون (present) have the prefix الـ (al-) like المقاولون does?

In Arabic, the subject of a nominal sentence is usually definite, so we use الـ in المقاولون to mean 'the contractors'. The predicate describing them is usually indefinite, so حاضرون does not get الـ. If we said المقاولون الحاضرون, it would just be a phrase meaning 'the present contractors' rather than a complete sentence.

Does the word الاجتماع (the meeting) change its ending because of the word في (in/at)?

Yes. The preposition في (in/at) forces the noun following it into the genitive case. If you were pronouncing the full grammatical vowel endings, الاجتماع would end with a 'kasra' (a short 'i' sound), making it 'fi al-ijtima'i'.