The campaign has been launched to coincide with the 21st anniversary of the establishment of the Rainbows, which is open to girls from the age of five before they join the Brownies.
There are now 80,000 Rainbows, and one in seven of all six-year-old girls in the UK is a member. But there are 18,000 girls on the waiting list to join, so it needs volunteer leaders to set up new units, and more unit helpers.
"Rainbows is going from strength to strength," said Liz Burnley, chief guide of Girlguiding UK. "When it started in 1987 we had no idea it would take off so well."
The charity offers volunteers the chance to participate for only 12 hours in a year.