Learn Arabic (Modern Standard): يقرأ حارس الأمن الإجراء الأول قبل أن يصل الموظفون. - The security guard reads the first procedure before the employees arrive.

Because it comes right after أن. The phrase قبل أن (before) must be followed by a present tense verb in the subjunctive mood. For regular verbs like يصل, this means changing the final default damma (u) to a fatha (a), making it yaṣila.

The employees (الموظفون) are plural, so why is the verb يصل (he arrives) in the singular form?

In Arabic, when the verb comes before the subject, the verb stays in the singular form, even if the subject is plural. Since يصل comes before الموظفون, it stays singular. It only needs to match the subject's gender, which is masculine here.

Why does الموظفون end in '-oon' instead of '-een'?

Because it is the subject doing the action (arriving) in the second part of the sentence. Regular masculine plural nouns end in ـون (-oon) when they are the subject of a verb, and they change to ـين (-een) when they are the object or follow a preposition.

Why isn't there an الـ (the) attached to حارس (guard), since the translation says 'the security guard'?

This is an Idafa (possession) phrase: حارس الأمن literally means 'guard of the security'. In an Idafa construction, the first word never takes الـ (the). It becomes definite automatically because the second word, الأمن (the security), is definite.