Create a Summary field in your Expenses table that is a SUM of your Expenses field.
Go to a layout based on an "Expenses" table occurrence.
Place the new summary field on the layout somewhere (likely in a sub-summary part set to show when sorted by User_ID
Do a find for a User_ID AND within your chosen date range (eg. 1/1/17...2/28/17).
Sort by User_ID
Your summary field should give you the total.
Create a relationship from your user table to your expenses table where:
User_ID (pk) = User_ID (fk)
StartDate (global) ≤ Expense date
End Date (global) ≥ Expense date
Create a calculation field of type number in the USERS table with this formula:
Sum ( users_EXPENSES::Expenses )
Place this calculation on a layout based on the USERS table occurrence.
(optional): Place a portal on this layout based on the users_EXPENSES table occurrence.
Place your global start and end date fields on the layout.
Your calc field will give you the total
In addition, ExecuteSQL could be used to calculate such totals or a filtered portal with a summary field in the portal could be used.