By default, all matters are made public and shared with all team members. Adding a member to a matter takes that access higher, enabling them to receive notifications of activities on such matter.
To add or assign teammates to a matter:
Go to Matters on the left side-bar menu.
Choose the matter to which you'd like to add a teammate.
Click the + button at the right-hand corner, then choose the teammate you'd like to add from the drop-down menu.