If you're serious about making money off of this, though, I'd highly recommend some form of per-seat pricing.
Per-seat pricing aligns the usefulness of the app with how much people are paying. Would you really want (for example) a 100 person team to be only giving you 15 pounds a month for what is probably _days_ of saved administrative time per year? Off the top of my head $15/user/year seems like a good price to deal with vacationing. Though ideally that number is MAX_NUMBER_YOU_CAN_CONVINCE_PEOPLE_TO_CHARGE/user/year