Learn Arabic (Modern Standard): نجحوا في مشروع السلام.

The verb نجحوا (they succeeded) ends with the plural suffix ـوا. The final ا (alif) is completely silent. It is a spelling rule in Arabic used on plural verbs to show that the و (waw) is a plural marker indicating 'they', rather than just being a regular letter that belongs to the root of the word.

The translation says 'They succeeded.' Does the form نجحوا refer only to a group of men?

Not necessarily! In Arabic, the masculine plural form نجحوا is used for a group of men, but also for a mixed group of men and women. You would only use a different suffix if the group was 100% female.

In the phrase مشروع السلام (the peace project), why doesn't the word for project (مشروع) have 'al-' (the) at the beginning?

This is a possession phrase (idafa) that literally means 'the project of peace.' In Arabic, when you link two nouns together like this, the first word (مشروع) is never allowed to have 'al-' (the). It becomes 'the' project automatically because the second word, السلام (the peace), has 'al-' attached to it.

Does the verb نجحوا always pair with the preposition في?

Yes, this is a very helpful match with English! Just like we say someone succeeded 'in' something, Arabic uses the base verb نجح (to succeed) followed by the preposition في (in) to introduce the thing they were successful at.