What is the best calendar / event extension?
Would like to have calendar / availability sharing between users in order to find people's availability when setting up a meeting / event.
Would like something that integrates or synchronises with Google calendar / outlook if possible.
Is any of this functionality available in current / future versions of JomSocial ?

Also to add code do we need a developer's licence ?

Hello Andrew

JomSocial is created to be a community backbone software for Joomla, and that said, it does not focus on calendar functionalists at all. However, you have vast amount of Joomla components specifically made for that purpose.
extensions.joomla.org/extensions/calendars-a-events

As for developer license, it depends what you want to do. If you plan to install JomSocial at only one site, than no, you don't need the developer license. If you plan to use it on more sites, developer license would be more economic solution.