Learn German: Es gibt eine neue Regel im Büro. Alle Mitarbeiter sollen sie beachten. - There is a new rule in the office. All employees are supposed to observe it.

Both translations work! sollen often expresses a duty, expectation, or a rule set by someone else. Translating it as "are supposed to" captures this feeling perfectly. Since Alle Mitarbeiter (all employees) is plural, the verb is in its plural form sollen.

Does sie in the second sentence mean "they"?

Not here! While sie can mean "they" or "she", in this sentence it means "it". It refers back to eine neue Regel (a new rule). Since Regel is a feminine noun in German, we must use the feminine pronoun sie to say "it".

Why is beachten at the very end of the sentence?

This is the standard word order for modal verbs like sollen. The modal verb takes the second position in the sentence (Alle Mitarbeiter sollen...), and it pushes the main action verb (beachten) to the very end in its basic dictionary form (the infinitive).

Why is it eine neue Regel after es gibt?

The phrase es gibt (there is / there are) always requires the accusative case for the thing that exists. Because Regel is a feminine noun, the accusative form is simply eine neue Regel.