Nobody suffers disadvantage, and only the PC who can see has advantage on attack rolls.
Per "Vision and Light", PHB,
Darkness creates a heavily obscured area. ... A heavily obscured area ... blocks vision entirely. A creature effectively suffers from the blinded condition ... when trying to see something in that area.
Per "Unseen Attackers and Targets", PHB,
When you attack a target that you can't see, you have disadvantage on the attack roll. ... When a creature can't see you, you have advantage on attack rolls against it.
Per "Advantage and Disadvantage", PHB,
If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them...
Without knowing exactly how the PC is controlling the creatures, I am going to assume that the PC is playing as a Mystic from Unearthed Arcana.
Per "Psionics", "Telepathy", Mystic Class article:
You can telepathically speak to any creature you can see within 120 feet of you in this manner.
So all PC characters except the one who can see will have disadvantage on their attack rolls, since they can't see their targets, but they will also have advantage on their targets, since the targets can't see them. These will cancel each other, and the PCs that can't see will not have advantage or disadvantage.
The PC who is controlling the creatures will have advantage on his attack rolls, since he can see other creatures but they can't see him.
The creatures being controlled will be in the same situation as the PCs who can't see. The creatures cannot see the PCs, since Telepathy lets you speak to any creature, not allow them to share your sight.
As a side note, being unseen does not automatically make a creature hidden. Per "Using Each Ability", "Hiding", PHB:
When you try to hide, make a Dexterity (Stealth) check. ... An invisible creature can always try to hide.
Emphasis mine. If being unseen automatically granted the hidden status, an invisible creature would not have to try to hide, they would just already be hidden.