While traditional cinematic viewing is often characterized as a social experience, engaging with immersive stories is predominantly researched as an individual activity. This mismatch becomes increasingly problematic in social virtual reality (VR), where features designed for typical social activities (e.g., virtual locomotion, spatial and social context) can become meaningful levers for shaping the user experience. To address our limited understanding of these effects, we conducted three exploratory experiments (N$_1$=109, N$_2$=116, N$_3$=88) examining how spatial boundary type, size of allocated personal space, and group familiarity influence co-watching 360-degree stories in social VR. Results shows that small communal spatial boundaries heightened social presence towards others compared to individual boundaries, but also increased mental demand. Reducing the size of the personal spatial boundary increased social presence towards others but also increased disorientation and frustration. Finally, participants with higher group familiarity reported stronger attentional focus on the story, as well as higher spatial presence and social presence towards story characters, but also reported greater physical and perceptual demand. We discuss systematic tradeoffs between engagement and cognitive effort, and suggest practical guidelines for designing shared immersive narrative experiences.

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