How can I see monthly active users trending in Pendo?

One option is to navigate to People > Visitors. Adjust the Segment to "Everyone" and update to your desired date range. In the chart, select "Monthly" and "Visitors."

Another option is by using Data Explorer. Navigate to Behavior > Data Explorer. Select "Any Activity" as your Data Source > Select your apps > Adjust the data range and select "Monthly" > Adjust the Segment > Click "Run."