All tables are defined as Tables.
To define a Table, you select an area, or you activate any single cell in existing table, and then select Table from Insert Menu. You can then also specify a name for created Table (by default it will be Table#). When you activate any cell of existing Table, a Table Design Menu will be available with some options.
In Excel formulas, you can refer to components of Table (Header row, Table datarange, column datarange, an active cell in current row, etc., using special syntax.
When you add new row of data into Table, then usually all formats, data validations, formulas etc. are automatically expanded to new row(s).
When you edit Table name, or the name of any column in Table, all formulas in Workbook referring to this Table are adjusted accordingly;
When you refer to Table column, or to specific header, or to value in current row of Table, this reference doesn't depend on position - you can drag the column to another position in Table, and all formulas (except ones referring to certain column number) will work correctly.

To ensure that no typos are getting into DataEntry table, data validation lists are used whenever possible. The Tables on sheets Participiants, EventCategories, and Retreats are for registering such values, and are sources for those data validation lists. Excel recognizes as sources for data validation lists:
1. The list of values entered directly on creation;
2. The reference to cell range ON SAME SHEET;
3. A defined Name (Name Manager from Formulas Menu).
I used 3rd option and defined a column in appropriate registry Table as Named Range (lParticipiants, lEventCat, etc.) - this ensures those names will be dynamic (whenever a new row is added to registry Table, this option will be available in matching data validation list automatically).

Instead of SUMIF/COUNTIF, I prefer SUMIFS/COUNTIFS, as they allow any number of conditions. And as they work with single condition too, there is no reason to use both of them - so when in future there will be a need to add a 2nd condition to single one, you don't have to edit the rest of formula.

The rest is Excel as usual. You can replace Table syntax in formulas with regular range references, and all will work - until you add a new row into DataEntry table