I've participated in design of application similar to that (it is already on the market): voice, video, camera desktop sharing and textual chat. First version was made in 9..11 months: core UI team - 3 developers for desktop. 2 developers for mobile clients, one for each platform. Server side 3-5 as it supports video transmission for 1000 and more participants. Platforms: Windows, Mac OS, Linux, Android and iOS. Small, manageable and so effective professional team.