Whiteboards and paper allow for any kind of notations and are easy to use. Requirements engineers love to use them in creative requirements elicitation and design sessions. However, the resulting diagram sketches cannot be interpreted by software modeling tools. We have developed FlexiSketch as an alternative to whiteboards in previous work. It is a mobile tool for model-based sketching of free-form diagrams that allows the definition and re-use of diagramming notations on the fly. The latest version of the tool, called FlexiSketch Team, supports collaboration with multiple tablets and an electronic whiteboard, such that several users can work simultaneously on the same model sketch. In this paper we present an exploratory study about how novice and experienced engineers sketch and define ad-hoc notations collaboratively in early requirements elicitation sessions when supported by our tool. Results show that participants incrementally build notations by defining language constructs the first time they use them. Participants considered the option to re-use defined constructs to be a big motivational factor for providing type definitions. They found our approach useful for longer sketching sessions and situations where sketches are re-used later on.