Immersive technologies like Augmented Reality (AR) have a promising potential for group activities. Collaborative AR systems allow co-located and/or distant users to create, visualize and manipulate together a large variety of virtual content in order to take actions and decisions. However, contrary to traditional 2D interfaces, there is currently no standard interface or generalized interaction technique for AR. This paper explores an approach for triggering commands linked to AR content. We propose MarkAR, a system combining virtual thumbnail representations of AR content (called vignettes), a microgesture and 3D mid-air mark system inspired by marking menus. Experimental results from a qualitative study highlight that MarkAR offers a great usability, subtle interactions and an easy way to trigger both discrete and continuous commands in AR.