I am attempting to find a way to lookup data from multiple rows of one
spreadsheet to populate cells on another. This is similar to the
vlookup function in Excel, but I need to look for every instance of a
string and append them together in a cell.

Here's the layout of the spreadsheets

Sheet1 (Source)

Column A Column B
Dog Ralph
Dog Spike
Cat Whiskers
Cat Fluffy
Cat Muffy

Sheet2 (Destination)

Column A Column B
Dog
Cat
Hamster


Thus what I'm looking to do is lookup the value in Sheet2 Column A in
the rows of Sheet 1 and populate Sheet2 ColumnB. My desired end
result for Sheet 2 would look like this:

Column A Column B
Dog Ralph, Spike
Cat Whiskers, Fluffy, Muffy

The actual worksheets are a couple of thousand rows long.

I've written a bunch of Windows system scripts in VB scripting, so I'm
familiar with how to script, but I've never done anything with VBA or
VB and multiple worksheets.

Can someone give me some pointers on how to proceed?

Thanks,

Andy