(CNN) It used to be that Moms Demand Action for Gun Sense In America, created the day after the 2012 Sandy Hook shooting, could count on maybe 20 people at their regular meetings.

The mass shooting at Marjory Stoneman Douglas High School in Florida has changed all that.

Now the group's leaders say they are scrambling to make room for 75,000 volunteers who have joined their ranks since last week's massacre.

More than 500 people showed up for a recent Raleigh, North Carolina, meeting, where organizers opened a spill room to accommodate the new volunteers. Instead of the usual 20 people or so at a meeting near Portland, Maine, 250 volunteers showed up.

Moms Demand Action founder Shannon Watts says she is seeing a new generation of youth ready to fight for more gun regulations. Tired of the lack of progress on addressing gun violence in schools, these young people are motivated to seek change, she said.

Demonstrators participate in a "lie-in" Monday in favor of gun-control reform outside the White House.