Up until version 1.7.0 there was a way to enable all developers of a company to work with SourceTree based on a corporate license:

https://confluence.atlassian.com/display/SOURCETREEKB/SourceTree+License+For+Corporate+Usage

From version 1.8 starting this possibility has been dropped and every developer has to register a atlassian account - and log in with that - in order to be able to use SourceTree.
This is even mandatory when managing repositories totaly unrelated to any atlassian product (local repos, repos from other services than bitbucket).

This kind of philosophy may be a nice tracking feature for Atlassian, but it's not acceptable for a companywide use.
I will not force every single developer to create a atlassian account and i m sure other companies won't do that too.

I highly recommend to take a step back or you will lose a large userbase (or it just sticks to very old versions - 1.6 series was the last real stable!).