The distinction between formal and informal forms of address in both spoken and written Italian is more relevant in the singular, Lei versus tu, and much less in the plural, since voi may be used in formal situations as well.
Loro is only used in place of voi when addressing (very formally) two or more people, if the speaker wants to signal distance from the addressees, and a very high degree of formality.
Singular
tu (“you” informal)
Lei (“you” formal)
Plural
voi (“you” plural, both informal and formal)
Loro (“you” plural, only in very formal settings)