I am wanting to take a customer list and have it return a true or false value
if the number is found on another list.

Specifics- I have a master spreadsheet of all my customers sorted by their
customer number.

I have another spreadsheet that lists only those customers that have made
purchases online.

I want my master customer list to say "Webstore User" if their customer
number is found on my list of webstore users.

I feel like this would be some combination of a lookup and an if then
statement, but can not figure out how to do it.

Thanks for any help!!