Reports can be assigned to either individual team members or to groups.
At the bottom of each report, above the comment box is the main "actions" drop-down menu. It will often default to "Add comment" when you first open the report. Click and select "Assign bug" from the drop-down, then enter the username of the assignee or the group. The comment box will turn pink, indicating that the assignment is an internal action, visible only to your team (not the reporter).
Click the green "Assign bug" button to complete the assignment.