Hi,
When Messages on the Mac starts up for the first time you are encouraged to add your Apple ID for the iMessages "registration".
Depending how you go to Mountain Lion you may have iChat IDs listed in System Preferences > Mail, Contacts and Calendars and made active for Messages.
This should mean any AIM or Jabber IDs are already in Messages as Accounts.
From there it tends to depend on how you use the Messages window and the process of Starting a New Message.
You can use the New Message icon that then makes the Blue + icon appear on the right.
This in turn can be used for Contacts or Buddies.
Or you can just type in the "To" spot and then chose the option you want that refer to that person.
Example of typing "ralph" in to the "To" spot.
It lists iMessages IDs and thinks any telephone number is a potential for iMessages.
My List is actually longer than shown and several Jabber accounts are listed as "Google Talk" like the one shown.
Messages will not switch Services mid chat to so Screen Sharing per se.
If you have the iMessage contact listed and are sending iMessages then using the Video icon will invoke FaceTime and not the older iChat Video that a Google Talk/Jabber or AIM Chat will do.
However clicking the Video icon will show you FaceTime and other Accounts Options (Depending on the info on the Address Card)
Google Info (not linked to and Address Card)
As you can see I prefer to use the actual Buddy Lists that are the background to this pic.
For me having the Buddy List displayed offers an One Click start to a Chat.
The icons below a Buddy List can be used to start the Chat type you want.
I will come back with another reply about Screen Sharing.
8:30 PM Friday; August 30, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously