Private lessons are $40 an hour, plus $15 for each additional person.

Admission: $11 for family of three, $6 each additional person.

There is a charge of $1,000 a month for an additional person.

That unit has an $800-a-month charge for an additional person.

Private lessons are $40 and $10 extra for each additional person.

Each cabin can sleep up to four at a cost of $10.85 per additional person.

Network effect - every additional person makes the organization stronger.

After this point, each additional person has less of an influencing effect.

As a group gets larger adding an additional person has less effect on its characteristics.

"It's worth it even for one additional person to volunteer."