For your friend:
In the messaging preferences on the phone, remove the appleID (email). This way the phone will only get texts to the phone number.
In the iMessage preferences on the computer, add the appleID (email).
When you pick your friend to message, make sure you send message to his appleID (email), rather than phone number and send message.
It is not about using the iMessages account within Messages to do what you did before.
The iMessage account cannot Video Or Audio Only chat and therefore cannot Screen Share.
However your Apple ID may also be the AIM valid Name you were using in iChat and it may be the same for your Buddy.
In Messages > Preferences > Accounts do you list anything other than the Bonjour Account and the iMessages Account ?
In System Preferences > Mail, Contacts and Calendars do you have the Screen Names and IDs you were using in iChat enabled for Messages - this may include your Apple ID if issued by Apple. (@mac/com @me.com and @iCloud.com endings)
If they cannot be enabled here you will have to add them in Messages as you would in iChat.
In Preferences > Accounts press the + button at the bottom of the List
In the top item in the Add account pane use AIM for AIM and AIM valid names.
Google and Yahoo are self explanatory as is Jabber.
This will add an item to the Window menu of the App called "Buddies"
It has keystrokes but using these or selecting the line will show your the "Buddy" List.
At this point if there are multiple accounts including the Bonjour Enabled (logged in( then the list is a combined one.
Go to the Preferences > General Section and Untick the line about gathering all accounts into one list.
You should now have separate Buddy Lists as you did in iChat.
These should have the Buddies you had in iChat in the various Lists. (if you had multiples)
Your Screen Sharing Buddy should be in one of the lists and the same methods of Starting Screen Sharing will apply.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
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