Take a Staff of the Magi for example. It can cast invisibility, which normally requires concentration.
Can the staff be used to cast invisibility on multiple people at once, or can it only have 1 up?
DMG: Activating an Item said:Spells
Some magic items allow the user to Cast a Spell from the item. The spell is cast at the lowest possible spell level, doesn’t expend any of the user’s Spell Slots, and requires no Components, unless the item’s description says otherwise. The spell uses its normal Casting Time, range, and Duration, and the user of the item must concentrate if the spell requires Concentration. Many items, such as potions, bypass the casting of a spell and confer the spell’s effects, with their usual Duration. Certain items make exceptions to these rules, changing the Casting Time, Duration, or other parts of a spell.
If an item is able to bypass the concentration limit, it will say so in its description. See Blackrazor for an example.
Yes, either by not referencing a spell in the effect description, or by explicitly stating "no Concentration required".Can items bypass the concentration limit?
I realize you mean "can I maintain more than one casting of the spell?" and there's the answer no.Can the staff be used to cast invisibility on multiple people at once, or can it only have 1 up?
Unfortunately using the ring counts as casting a spell (see the Activating a magic item quote above) so the example was a bit unlucky.Yep, check the specific item description. If the item says that it allows you to cast a spell, it's a spell with concentration restrictions.
An item like the Ring of Invisibility, that doesn't refer to casting the spell...you should be okay.
RING OF INVISIBILITY
Ring, legendary (requires attunement)
While wearing this ring, you can turn invisible as an action. Anything you are wearing or carrying is invisible with you. You remain invisible until the ring is removed, until you attack or cast a spell, or until you use a bonus action to become visible again.
Unfortunately using the ring counts as casting a spell (see the Activating a magic item quote above) so the example was a bit unlucky.
Yes, you don't need to concentrate, but that doesn't help for this particular spell since Invisibility specifically ends when you cast a spell.
Meaning that if it had been most other spells, you could have cast that spell on someone else and then use a Ring of That Other Spell to also buff yourself.
It's just for Invisibility the example doesn't work. The idea itself is sound.
That might well be true, but don't forget potions.The only example I'm aware of where a magic item will concentrate on a spell for you