Github not only allows the developers to develop projects but also provide a well organized environment to communicate with each other.
You can create issues for your collaborators and assign them for particular tasks.
First you need to log in to Github.com.
Then click on your project for which you want to create issue.
Then click on Issues.
Now click on New Issue to create an issue.
Write down the issue and you can assign someone for this task.
You can set milestone too for the task.
At last click on Submit new issue to complete the process.
Now the issue is created and your collaborator will be notified for this task.