HI,
This is a known issue with no current fix at present.
It is effecting people globally and from iChat 5.0.3 (at least) upwards through all versions of Yosemite's version 8 (it is not clear due to numbering changes if Messages in Yosemite has been updated).
It is also effecting both AIM and Jabber (Google) connections.
Screen Sharing is also effected, using those same account types (the same ones iChat used before Messages).
Yosemite's version of Messages can Screen Share in iMessages and I have not seen posts to say this is not working (it works differently to the old "iChat way").
8:49 p.m. Friday; July 31, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
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