My player is also a werewolf and the Harbinger for the companions. I would've like to have the option of recruiting Sinding, a werewolf, to the companions. Since the circle are werewolves themselves Sinding would have plenty of help to control his werewolf desires and so on. I would also like to have the option to cure him from his werewolf disease by using one of the heads of the glamor witches.

An addition after Sinding escapes from jail you meet him at Bloated Man's Grotto. I would like to share him my little dark secret in a dialog as well and to hear his reaction. Maybe like "Oh you're a werewolf too?!?!" or "you and I are the same kind"

Bethseda make this RP more interesting and less boring.

I'd like to set him up with Aela. I've cured my character and everyone else in the Companions, but I gave her the Ring of Hircine and imagined mentioning that male werewolf living in that cave... In case she wanted a fellow werewolf to talk to

Aela would just end up walking around in the Saviors Hide after poor Sinding ran in front of her arrow while hunting.
Skjor and Aela would kill him for Hicine, they want to go to the Hunting Grounds when they die.

That would have been cool to let him join the Companions so he has people that know his condition, but they wouldn't be able to teach him how to control himself because he is forced to transform at the full moon. But Sinding could not be cured of his Lycanthropy by putting a Glenmoril Witches head on the fire because he did not get his Lycanthropy from the pact the Companions made, he got infected normally.

You might want to explain things to me because I'm somewhat confused. Why can't Sinding be cured from his Lycanthropy? Don't all werewolves have the wolf spirit in them? Some go feral because they're unable to control the wolf spirit inside them thus making them into 100% werewolf? I remember Aela mentioning something like that during a questline where we approached a dead werewolf. She mention that it could've been anyone.

I don't think it matter how, when and where Sinding got his werewolf disease from as long he can defeat his wolf spirit to be free from Lycanthropy and from going to Hicine hunting grounds.

Sovengarde or Hunting grounds...Mead for eternity or slaughter for eternity...Drunk Nords or Werewolves...i would stick with lycanthropy. Sinding does not seem the kind who wants to hunt so companions dont seem to be his thing

The Glenmoril Witches blessed the Companions with the spirit of the wolf, therefore only Glenmoril lycanthropes can be cured with a Glenmoril Witch head. Normal lycanthropes may be incurable without divine interference. Maybe Clavicus Vile will be more generous this time.

As Pink has stated, the known cures for a Feral Werewolf require,
A. a living Glenmoril Witch with the knowledge of what to write on a scroll<--- not an option for most players who are Harbinger.
B. a Mage who has knowledge of the ritual used to cleanse the soul of a normal Werewolf<--- this option also requires the slaughter of an innocent by the infected person.
I am going off of what the requirements were in Morrowind.
The ability to cure Feral Werewolves like Sindig may be added as DLC, but more than likely it will be modded content.

Just as bradleyh said, that is how you cure a "normal" werewolf, like Sinding. The Companions are a special form of Lycanthropy because they were blessed with the power by the Glenmoril Witches, rather than being infected, so they are the only ones that can be cured by taking the heads of those witches and throwing them on the fire, and then killing your spirit wolf.