WASHINGTON — After President Trump’s inauguration, Debbie Matties felt helpless.

The next day she attended the Women’s March, and reflected on her past work volunteering on Democratic campaigns. But Ms. Matties, a Washington resident with little say in Congress, felt left out of the democratic process.

So she joined a Facebook group of other disillusioned liberals with an eye toward constituency advocacy — if their voices could not be heard, at least they could amplify others across the country.

“We started contacting friends and family who don’t live in the District of Columbia saying: ‘Hey, I live really close to the Capitol. Send me a letter by email and I’ll print it at my house and bring it up myself because I just feel so frustrated and I don’t know what to do with myself,’” Ms. Matties said.

That ragtag group of volunteers would soon become Herd on the Hill, an organization with more than 300 members who have spent their lunch breaks and after-work hours delivering more than 12,000 letters to congressional offices on behalf of constituents seeking to inject a human element into the issues roiling the country.