Does your business already have existing investment in Cloud productivity? If so, you more than likely use either Office 365 or Google Apps - the two leading Cloud productivity suites.
Timestamp allows simple configuration of single sign-on (SSO) with either of suite. Simply follow a trust granting process within Timestamp to allow on-demand synchronisation of your user profiles whenever you want.
Single sign-on simplifies administration overhead. Manage users in your preferred tool, maintain existing password policies and reduce needless data entry. Enabling SSO also keeping things simple for your team members; there are no new passwords to remember or barriers to entry. If your team members already have a set of security credentials that they use on a regular basis then why make things complicated?
If you want to add someone who hasn't, or shouldn't, be added directly to your productivity suite then just add them to Timestamp. Mix and match SSO and direct Timestamp users to meet your specific business requirements.