Cause:
This is a common issue when the host forwards the host confirmation email with the link to start the meeting, instead of inviting attendees by email. The attendee invitation email contains the link to join the meeting, without needing to log in to the site.
Solution:

To resolve the issue, the meeting host can try any of the following:

• Forward the attendee email invitation instead of the host invitation (the host must add themselves to the attendee list in order to receive the attendee invite email.)
• Add the attendee to the invitation list when scheduling the meeting to ensure that the attendee receives the proper notification and link directly from Webex.
• Invite the attendee from inside a meeting that is already in progress (see article: Invite People to an Ongoing Webex Meeting for detailed instructions.)

• WBX71898 - How Do I Contact the Host of My Meeting?