You can assign one or more members to a task. Access the detail of the task in question by clicking on it. On the "Who" tab, select one or more members in the "Assign a member" value list.
You also have the option to specify one or more actions for each member. To do this, click on "Add an action" and enter your text.
Note 1: Validation of an action does not result in the validation of a task.
Note 2: To manage the assignment and access of members to a project, see the dedicated article.