## 1 Introduction  
Immersive virtual reality (IVR) is considered a promising space for delivering authentic and vivid experiences [1, 2]. IVR enables students to gain a presence in a highly immersive virtual environment with the capacity to afford high-fidelity simulations, rich sensory stimuli, and multimodal interaction [3]. The sense of presence describes the illusive perception of being present in an artificial or remote environment [4], and immersion is often referred to as a feature of VR that provides a convincing illusion [5]. The past decade has witnessed IVR being applied in various domains, such as entertainment [6], medicine [7], engineering [8], consumption [9], and education [10]. However, previous studies on IVR have mainly focused on individual experience, and little is known regarding users' collaboration experience in IVR [11]. The implementation of collaboration in IVR is hampered by several challenges, such as geographical distance, temporal distance, and perceived distance [12]. Collaborative work can be classified according to the modalities of the space and time taxonomy as follows: synchronous co-located, asynchronous co-located, synchronous distributed, and asynchronous distributed collaboration [13, 14]. Both verbal and nonverbal interactions, which are pivotal for effective group work, are diminished by the asynchrony of time and space during collaboration [15]. This reduction in interaction negatively affects trust between team members and increases the perceived distance between individuals, ultimately undermining the collaborative experience and effectiveness [16; 17]. For instance, distributed virtual collaboration has been explored by several studies [18; 19], where communication primarily takes place through formal texts. However, this mode of communication still poses challenges to establishing confidence and trust within the virtual space. Synchronous co-located virtual collaboration is an innovative mode characterized by users entering the virtual space while being physically present in the same location, which enables direct communication between users without any mediation [14]. The social and emotional experiences between collaborators are enhanced through unmediated interaction, which are strongly associated with learners' engagement and performance [20]. For example, [PERSON] et al. [21] designed a virtual mall where two players stayed in the same physical space and wore VR head-mounted displays (HMDs), which helped them communicate naturally to find goods collaboratively or competitively. Positive affective outcomes, such as increased enjoyment, intrinsic motivation, and social presence, could be facilitated by immersive and interactive experiences [22], which can benefit learning performance by focusing learners' attention on the task and enabling generative cognitive processing through more frequent and natural social dialogue [23]. However, synchronous co-located virtual collaboration has rarely been explored owing to its additional technological requirements and a lack of design principles. First, the commonly used outside-in tracking technology that enables multiuser co-presence in IVR has proved to be problematic [24; 25]. It is susceptible to glitches caused by occlusion and lighting issues, and it also requires many cameras and sensors to be mounted in advance, which increases the total cost and preparation time. Second, a list of validated design assumptions for promoting effective collaboration in IVR is still lacking owing to insufficient design cases and intervention studies in the literature. It remains unknown whether effective design principles for collaboration in brick-and-mortar settings still apply to IVR spaces. It is imperative to overcome these two challenges to enable collaboration in IVR so that students can harness its pedagogical benefits, including social dialogue [26], shared inquiry [27], and participatory equity [28].