If someone is unsure they're able to make it (eg. needs to check their calendar), they can be tentatively rostered to the event.
The person being rostered will be reminded via email that they need to confirm their shift, as well as via SMS if they still haven't done-so a week away from the event (which they just need to reply via SMS to confirm they're able to attend).
Confirmation
The person rostered can confirm themselves via the home page.
Confirmation can also be done when viewing the event details;
SMS Replies
Depending on your settings, you might have the system send an SMS to staff rostered tentatively as a way to reduce the time required for staff to confirm they're able to assist for a shift. These messages will come from a random set of phone numbers, in order for us to then receive responses your staff have sent, and match those up to the relevant event they're referring to.
Where replies are expected, other numbers such as your company name, or a central company number can't be used as the system won't receive these responses (your company phone number will get sent these!)