You can ensure that the program always keeps families together on a global basis or on a family-by-family basis.
You can ensure that only specific families are never scheduled without all of their members by going to the Families pane, opening up the families, and then checking the "Never schedule this family without all of its members" option. Note that when this option is turned on, the scheduler may not be able to give some members of the family as many services as they should otherwise have, because other members in the family should not be scheduled as often. Since the scheduler will never split up the family, it can't schedule the members who should be scheduled more often for their additional services.
You can also ensure that family members are always kept together just in a particular ministry, which will not adversely affect distribution. For example, you can specify that all couples who are both Greeters should always be scheduled together in that ministry, but may be scheduled apart when serving in their other ministries. To set this up, you would go to the Ministries pane, open up the Greeter ministry, and then check the "Never split up family members in this ministry" option.
To ensure that the program never schedules ANY family without all of its members present, you can check that option in the Scheduler Settings dialog, which you can access by choosing "Scheduler Settings..." from the Schedule menu of any open schedule or schedule skeleton window. However, doing so will make it harder to fill the entire schedule.