Companions & Attunement

Creatures summoned or otherwise under a player's control share their attunement slows with you (the character), with their attuned items counting against your own. This includes creatures such as familiars, beast companions, simulacrum, etc.

Hirelings, as they are technically their own entities and not an extension of you, are not subject to this rule - but keep in mind that a hireling using magic items falls under "skilled hirelings" and prompts a significantly higher upkeep than normal.