Secure and timely delivery of messages for safety applications in Vehicular Ad Hoc Networks (VANETs) is crucial to prevent fatal accidents. However, security and delay are two competing metrics since security brings additional processing overhead, increasing the packet delays. While symmetric-key based security techniques can be more efficient as opposed to public-key cryptography (PKC) in terms of delay, they introduce significant key maintenance overheads with the increased number of vehicles in VANETs. To alleviate this overhead and take the advantage of faster processing, we exploit the natural group behavior in VANET applications. We propose a delay efficient authentication scheme for VANETs which is based on group communication. Groups are created and maintained geo-dynamically led by leader vehicles. Since the data communication within the groups will be dominating the overall packet traffic, we utilize symmetric-key techniques within each group which is handled by the group leader. Group creations on the other hand are less frequent events and thus done by PKC. We analyzed the security and efficiency properties of our proposed scheme to show its spueriority with respect to other existing techniques.