Q: screensharing via yosemite

Not a lot of information.

System Preferences -> Sharing -> Screen Sharing

If other Mac is on the same local area network, then your system should appear in the Finder sidebar.

If you need remote access, then enable Back-to-My-Mac

<Set up and use Back to My Mac - Apple Support>

An alternate approach is TeamViewer.com which is cross platform.

Both of these approaches end-to-end encrypt all data transmissions.

Hi,

Also Messages > iMessages account.

Start a conversation with someone then use the Details Button top right. (you need the interlocked rectangles)

This invokes the Screen Sharing App (Same as System Preferences > Sharing > Screen Sharing does)

The "iChat Method" for AIM  or Jabber Buddies  no longer works.

In Yosemite and later it has been turned Off.

However the Video or Audio Only chats also don't work as there seems to be something wrong with the SNATMAP server that connects the chats.

(It works for LAN connections like Bonjour in Messages or iChat between two Macs or even using two AIM Screen names on both)

To be clear from Yosemite there are technically two methods you can employ from within Messages.

The original Method from the iChat days.

And the iMessages invocation of an outside app (like it does with FaceTime), in this case the Screen Sharing App (which is hidden away in Core Services).

8:51 pm      Wednesday; January 27, 2016