If you're in a situation where you need to finalize the group in a hurry, the best solution is usually to edit your own commitment to make up for the difference.
When you click "Edit Details," you'll be able to adjust your share to boost your group up to the tipping point. For example, if your group has a minimum of 8 people but only 5 (including you) are committed so far, you can edit your share to four spots instead of one.
This lets you complete the group, but you'll need to start a second Pay By Group or collect cash from the people who you're covering in order to get paid fully.
Another alternative would be to change the tipping point directly, by either editing the cost of the group or editing the minimum number of people needed to complete the group.
Any increases to cost or decreases to the minimum number of people needed to complete the group will have the effect of raising each person's share price. This will force everyone in the group to accept the cost increase. For more about this process, see this article.