I use Doodle a lot. It is possible to tie Doodle in with Google calendar (which also means I see it in my calendar on my Android smartphone). You can set up available time slots in Doodle that can only be filled by one person. Once it is filled it becomes unavailable to others. There may be similar tools but I have only used Doodle and find it extremely efficient. The only drawback are those who insist on having access at some other time but there will always be a few of those, and, it is easier to accommodate a few exceptions this way. I use the method for the 50 grad students in our department, with who I need to meet once a year. I schedule hours at the beginning of the term and ask them to book as soon as they can.
So using Doodle with Google calendar or some similar combination is a useful tool.