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To find staff, view the event details, and then select 'Find available staff' from the dropdown box 'Tools' menu at the bottom top-right of the page.
From here, you'll just need to select the appropriate staff, and then either add them to the roster, or send them a message - possibly requesting they view the event details, via the dropdown box at the bottom of the screen.
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An optional message can also be included with your Email or SMS if needed.
Automatically Finding Staff
Within the 'Configuration' page under the 'Settings' section, you can select 'Attempt to find staff for your event' which will automatically send emails to those people whom are available to attend your event. You can have the system attempt to find people anytime between 1 day prior to the event, through to 3 months prior to the event.
To turn off this facility for a particular group, first hover over the 'change' link at the top of the page, and select the group that you would like to affect.
Once the group has been selected, click on "Don't send any requests" for the 'Attempt to find staff for your event' option.