This page will explain how to add a feature layer to a Collector web map in ArcGIS Online (AGOL)
In AGOL, navigate to your Group page
Press the "Content" tab in your Group
Find and select your Collector Map (Organization Name)
Press the "Open in Map Viewer" button
See the Map Viewer with the Collector Map (Organization Name)
Press the "Add" button
Choose "Search for layers" Note: There are different options available, including, from file
Type in the name of the layer in the search section. Note: examplepolygon was used in this example
See that the new layer example polygon has been added and is present in the Legend
Press the "Save" button