You can create and invite clients from the Clients page in your sidebar.
On the Clients page, simply click +New in the upper-right corner. This will open a modal allowing you to input the client's full name, email address, and company (optional).
You can also choose to send an email invite immediately using the checkbox next to Send email invite. This is box is checked by default.
Note: If this box is unchecked, the client will be created but they will not be invited immediately. You can send them an invite at any time by clicking the action button (...) to the right of the client and choosing Send invite from the dropdown. You can also select Copy invite link and send the invite manually via whichever method you prefer.