To make your life easier, user management in Chili Piper is done via Salesforce. If a new user is added in Salesforce, it will show up within 15 minutes in Chili Piper. If a user is disabled in Salesforce, it will be disabled in Chili Piper automatically as well.
If a user doesn't have a Salesforce license, you can add him as a free Chatter user (he doesn't have access to Salesforce record, read more about free Chatter users on Salesforce site)
A user doesn’t become active unless he/she activates their account by logging in to Chili Piper. See how a user activates their account in the user section
Is the user still not showing up?
It takes up to 15 minutes for Salesforce user data to be pushed in Chili Piper. If you're already passed those 15 minutes, make sure your user is turned on in the correct workspace
Adjust user access including "master admin" privileges via Admin Center > Users
If it's been more than 15 minutes since you've added the user to Salesforce, you can have the user go login directly to Chili Piper at https://apps.chilipiper.com/login.html via Salesforce, which will force his/her user to sync into your Chili Piper environment.