Basically, there is no seperate user web portal, your users login to the same place as your admin account.
If you're using WebEx Scheduling (see Deployment Options), then users will login via your webex.com URL (ie companyname.webex.com). Your WebEx account team should have provided you with the details for this. If you use CUWL, you would have started this WebEx process by filling out this MP8/WebEx provisioning form.
Basically when users click the 'Host Login' button on your webex site, if you're using MeetingPlace for Directory Service or Integration (see here) you'll be taken to the same MeetingPlace page that you currently login to with your admin account. If you login with a normal user's username and password, this will then be passed to WebEx and you'll be logged in (if the user has never logged in before, the WebEx account is created at that time).
Similarly, if a user logs in to your MeetingPlace website (where you login as admin) with their own user details, they'll be diverted to your webex.com site and be logged in.