If you have MyGuest setup and enabled for your property, then you can grant access to any User already setup and active in MyPMS.
To grant a User access to MyGuest, go to https://mypms.bookingcenter.com then SETUP | PMS | USERS. Click on the User ID of the User you wish to grant access, and click Edit. In the My Guest Access field, choose "YES" and click Save.
The User will immediately be granted access to MyGuest (either https://manage.bookingcenter.com/ and/or the Android/iOS apps) and can login to the Management console with their MyPMS User ID and password.