How do the sets work that call additional followers to your side?

I do not personally have the set, but based on conversations in this thread, I will say the following regarding Asheara's Vestments:

• When followers are summoned, they will have the gear and skills you actually equip them with (so the dps and tankyness are dependent upon how you gear them)
• The followers are up very often if not practically all the time.
• The followers are often all three out, though they do occasionally come in one at a time.
• The followers can be summoned even in multiplayer games.

Relevant quotes:

The damage they add is however much they have when equipped. They are your actual followers and their equip. I ran with a witch Doctor who had them out almost permanently.

Brock - yes, I was playing last night in adventure mode doing Rifts on public games. All 3 followers come out; however sometimes it seems they come out one at a time.

If you are playing a private game, and already have a follower in your party, only the other 2 followers will appear. But don't forget to equip your followers with good gear; I would advise that the followers at least do something in the ~100k damage range.

Just to add, I crafted 3 and got a ring of royal grandeur, so I now have the 4 piece set bonus. I timed it and did multiple experiments.

The duration of summoned followers is 30 seconds. The proc rate was significantly higher on AOE packs, almost guaranteed within the first couple of multishot casts, where it was somewhat lower on small packs and elite packs, but they still came out pretty reliably even after a few hits. I'd say you have a good chance of these guys being out 2/3 to 3/4 of the time, so it's a good set to get, especially if you can get your followers some good weapons, and get them the tokens that unlock all abilities

As I have crafted the Ashera's Vestments set I do have the bonus which calls the companions. I can't give detailed numbers but all 3 are added on any attack you execute (in solo and multi-player parties; not sure if any spell will trigger their spawn tbh) I think like every 2 minutes and stay 45+ seconds. I furthermore have equiped my companions to have 200k+ DPS each and for single-player this is a appreciable DPS boost. In multi-player groups the DPS boost is not that significant in my opinion - especially when you group with OP barbarians or one-shotting mages, but it still adds DPS or a bit of crowd-control (taunt from Templar, ...) from time to time. Even items with certain effects work on companions.

I haven't noticed that only one or two of them appeared - I always got all 3 at the same time in multiplayer - in single-player usually there is already one around.