It is possible to have multiple app users connected to the same Spacetalk watch without any additional costs - however, only one primary account holder can 'pair' with the Spacetalk watch.
To pair the primary account, you can find helpful instructions here
You can add other app users by Device Sharing, which will provide the same level of access as the primary account holder, unless otherwise set by that account holder in their settings menu.
To share access, please follow the steps below:
Primary Account Holder
Open the Spacetalk app and press the '+' Symbol at the bottom left of the screen
Select the profile name of the device for which you wish to share access
In the new pop-up menu, select 'More'
Press "Device Sharing", then press the plus in the bottom right corner
Enter the email address of the person you wish to share access to then press send
Secondary Account Holder
Make sure the second person has downloaded the Spacetalk app, and created their own account under 'New Family'
Once done, navigate to the '+' Symbol at the bottom of the page and it should have the profile name under 'Family'
Press on the profile name and accept the sharing request.