Screen Sharing: How do I share my screen with my friend's Mac?

Messages – Screen share - Yosemite

If the other user is not on 10.10 you can setup Google accounts in Messages at both ends & use that to share screens.

I don't believe you need to activate screen sharing in System Preferences that is for local access.

HI,

Whist his Screen Sharing item might also be On does he list any restrictions ?

As you can see by your Pic it could be set to "Adminstrators" but these would be Admins on his computer not yours.

It is also possible he has set a specific person.

This would also have to be some who has a Mac User account on his computer not a Contact's Name.

What the iMessages side does here is invoke another app like it does when using FaceTime for Video or Audio Chats.

This means the other app has to work.

However there are part of the Messages use of the Screen Sharing app involved in the connection process that are some what easier than using the app itself on it's own.

The Screen Sharing app is part of Apple Remote Desktop.

It's engine as such is used my the Messages app in the "iChat type" video chats in AIM and Jabber accounts.

The app can be used on it's own by making VNC connections to the other end. (AS shown in a link from the Link Eric Posted)

However this involves the router allowing the ports and the other end also pointing them to the "right" computer.

From what you post though it does seem you have everything set correctly at both Macs.

That might leave issues with any router.

Is it using UPnP ?

This would allow the app to tell the router which ports to have open without having to manually set them.

8:57 p.m. Sunday; June 14, 2015

Hi,

for the Points

This can be the sticking point.

However if his router can do UPnP then this will allow the computer (via the app) to say which ports it wants open.

If he has a Apple Base Station of any sort Apple have a setting called Port Mapping Protocol that does the same thing.

In both cases you don't have to set ports.

10:01 p.m. Sunday; June 14, 2015

Does Messages log anything when you attempt to connect?

Open /Applications/Utilities/Console & look at the 'All messages' section (you may need to open the Log List via the toolbar).

When I attempt to connect it logs info (about supporting audio & finding the remote IP, that also happens on the remote end).

I find that it can be very slow sometimes, so it can appear to fail during the phase where the remote user accepts sharing.

I also find that deleting the account (or disable/ sign out) in Messages & recreating it can sometimes help (at least with Google chat accounts). You should get confirmation emails from Apple as devices are added so don't be alarmed by them 🙂.

If you can't make any progress consider using LogMeIn, GoToMyPC, Teamviewer or any other remote access tool to get started, that may allow you to tweak Messages into working.