Select a user you want to change permissions for, click on the "..." menu and select Edit permissions option.
First you need to define is this user an employee, a client or a contractor. Read here about different types of users.
Employees can be in one of the three roles:
Staff is a role with a basic set of permissions. Staff can manage tasks and task lists and collaborate on projects.
Staff can't access company settings, financial information or salaries.
It can't manage People and companies or see other people's time entries by default.
This two permissions need to be added manually to a member of a Staff.
Manager is a role that manages projects/budgets and have access to all deals and budgets with relevant financial information.
Manager doesn't have access to company settings or salaries.
Admin can manage everything, including creating user accounts, managing salaries and general organization-level settings.
Clients can manage tasks and task lists and collaborate on projects. Clients can also get access to budgets and timesheets.
This is a global permission setting for a specific client. To give a client an access to a specific budget, you need to go to Projects, select the budget and turn Client access on in a Timesheet tab.
Contractors can't get any special permissions, they can only collaborate on Projects, manage tasks and task lists.