You can invite your external clients to your projects. Clients will only have access to the tools you select when inviting them. For most of the tools clients have read-only access, which means they can view your work and progress. However, clients can leave comments and participate in the chat and the discussions. They can also create a discussion.
Inviting clients is 100% free. The amount of clients you can invite depends on the plan you're on.
You can invite clients the same way you invite your other teammates, via the sidebar. Click on "Invite people" and in the top select "client". Fill in the client's email, the projects you want to invite the client to and select the tools you want to client to be able to access. Then press "Send invites".