To invite someone - regardless of whether you are inviting a regular or a Read-only user - you should have the necessary permissions. If you are a Read-only user yourself, you will not be able to invite anyone.
If your user type and role allow you to invite others into the workspace, to send an invitation, go to the "Invites" page.
On the invites page, click "Invite new member":
When setting up an invitation to invite someone as a Read-only user, check the respective box:
If the "Read-only" checkbox is checked, it does not matter which Role you select above: Read-only type will override any role permissions.