Once you have your clients signed up in Productive, it's time to give them access to projects you're working together on.
Type in the search the name of your client and select it from the list.
Switch to the projects tab and find a project you want to add this client to.
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. It can only collaborate on the project - see tasks and task lists, open new tasks, comment on tasks, close tasks, filter tasks, etc.
Take a look at what the client sees when added to a project.
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. Read more about it here.