An online system identifies a conversion of an advertisement when a client device associated with an online system user is within a threshold distance of a physical location associated with the advertisement. The client device sends obfuscated information identifying the user associated with the client device and the location of the client device to an independent third party when the client device is within a threshold distance of the physical location associated with the advertisement. Using information from the client device, the third party requests information from the online system identifying one or more groups including the user. Based on information identifying groups including various online system users, the third party generates information identifying conversions associated with different groups. The online system may determine effectiveness of various advertisements using the information from the third party identifying conversions associated with different groups.