User Centric vs. Team Centric Approaches
GitHub and Gmail each had significant traction with individual users before their products were widely adopted by enterprises. They each took slightly different approaches to how teams would be implemented as a result.
GitHub started by allowing individual users to create organizations through which they can invite existing GitHub users. For GitHub, if your company uses their hosted private repos, then your personal account is granted access to your company’s organization. A personal GitHub account can be a member of countless organizations. Of course, companies can elect to use GitHub Enterprise, in which case users will not use personal accounts (since the entire instance is private).
Google Apps Organizations
Google Apps saw a similar adoption pattern from consumers who then started using many of the tools for business. However, when Google Apps for Work was introduced it did not offer a way for a company to invite existing Gmail users to their organization. Instead, existing Gmail users were instructed to sign up for a completely new account through which they could access the Google Apps for Work organization. For the longest time you couldn’t actually be signed into more than one Google account at a time (now this is possible).