A Kilimanjaro open group tour is one in which anyone can join the climb. This means that you will join other climbers that have booked the same tour and it is not limited to the people in the group knowing each other.
Group tours tend to be cheaper when compared to private tours and they have a set schedule. Some operators require a minimum of two people to join the tour before the booking can be confirmed. All climbs have a maximum number of participants, so be sure to book in advance to secure your place.