You can notify a group of members at the same time in the main page's results list by clicking on a member's group color block to filter the list by group, then check each member's checkbox (or use the Check All button), then select Notify from the menu at the bottom of the results list, then press Submit.

You can also notify a group by selecting Groups from Last Visit™'s navigation menu, then, using the last line of options on the Groups page, select a group from the first drop-menu, select Notify All from the second menu, then press the Go button to execute the command. All members in the group you selected will be sent the default email notification and will be marked as notified.