Excel 2007 : LOOKUP to extrapolate data

I want to extrapolate data from spreadsheet ‘a’ to spreadsheet ‘b’ using the LOOKUP formula if possible. The common thread for the two spreadsheets is a column labeled ‘Unit Name’. Spreadsheet ‘a’ is the source document and lists units by activity – every time there is an activity for a unit name, there is a separate line entry. Spreadsheet ‘b’ is my summary sheet and lists all of the unit names, but only once. I want to populate sheet ‘b’ with the dates of the activities from sheet ‘a’, but I want the unit names listed only once, with the multiple activities (where applicable) populating spreadsheet ‘b’. When I try to use LOOKUP, the data it draws is bogus, I suspect because when it looks for the unit name in spreadsheet ‘a’ it finds several instances of unit name, each with a different activity for the name. Is LOOKUP capable of this action, or is there some other option?