According to various, but not necessarily all, embodiments there is provided an apparatus comprising means for: obtaining a propagation profile for wireless signals transmitted between at least two devices via a creeping wave along a user's skin; causing transmission of electromagnetic radiation towards a plurality of locations on a target user's body to obtain dielectric properties of their skin at the plurality of locations based on an amount of the electromagnetic radiation reflected from each location; determining whether the propagation profile correlates with a realizable creeping wave along the target user's skin, the realizability being based on the obtained dielectric properties of their skin; and forming an association between the target user and the at least two devices based on a strength of correlation between the propagation profile and a realizable creeping wave.