Once your clients have been added to Productive, it's time to give them access to projects you're working on together.
Go to your client's personal profile in Productive manually, or type in their name in search. When there, open the Projects tab.
You can add one client to as many projects as you need. If you want to remove them from a project, click on the x button next to a project.
At this point your client still doesn't have access to any financial information about the project. He can only collaborate on the project - see tasks and task lists, open new tasks, comment on tasks, close tasks, filter tasks, etc.
Access to financial information and time entries is a special kind of permission that needs to be granted separately from just adding a client to a project.