I am looking for information on how to share private photos on the main site as well as when using a mobile device.

Thank you.

I can give you the 'PC' way:

How do I make Photos 'Private?'
When you place a picture in the 'Main Pic' slot
it will allow you to designate it as "Public"
or "Private."

You do this by left-clicking on the photo.

To give someone access to the 'Private' images,
you merely right-click on the 'Private' photo
which gives you a drop-down menu, left-click on
'Properties' and copy the URL (link) of that
photo and send it via email to anyone you wish.

It stays 'Private.'

When you say send it via email, how about on this site, as i would prefer to send it on this site than use an email address...

The site email is the vehicle ;-)

Yeah, the site calls their messaging system "Mail" but it isn't really eMail it is an in-site messaging system.
As far as I know, you can't access, send or receive mail from M2 in Outlook or Thunderbird because there is no POP3/SMTP identities to associate.
You don't have an email address like yourname@M2.com, all your activity is done while signed into your account at this site.

The actual eMails you do get are from this site notifying you of something.
You can get an eMail from M2 saying somebody has sent you a message but you have to come to M2 in order to read that message.

A lot of people get confused over the eMail/Message thing.
Its normal and usually not a big deal.
I won't use off-site eMail until I know someone better and trust them, even then, I won't send them my primary eMail addresses. Rather be safe than sorry.