Does anyone have legal experience and know what the restrictions are to forking of the Mattermost project? I'm a programmer with experience in backend developm

Does anyone have legal experience and know what the restrictions are to forking of the Mattermost project? I'm a programmer with experience in backend development and security and would be willing to donate some time to work on developing a stronger community implementation of Mattermost - only I'm not sure what the legally correct way is to go about it. What I'm hoping to find is a FOSS sherpa to help me navigate setting up a new project.