If you are using Spatial Join in ModelBuilder then after you connect necessary ovals to the tool, double-click the Spatial Join tool to open its dialog. Then, go down to the Field Map control - it lists all the fields from the target and join feature classes. Find the field of the Join Features (points in your case), right-click on the field name, move your cursor to Merge Rule and select Sum from this context menu. When you run the model now you will see the values from the joined points are summed up in a field of the output. Let me know if you still have any issue.
Nobbir Ahmed
Esri Geoprocessing and Analysis Team