During group interactions, we react and modulate our emotions and behaviour to the group through phenomena including emotion contagion and physiological synchrony. This paper introduces a new interpersonal Weighted Group Synchrony approach based on Electrodermal Activity (EDA) and Heart-Rate Variability (HRV). Two datasets (AMIGOS and K-EmoCon) are explored, covering different group sizes (4 vs. dyad) and group-based activities (video-watching vs conversation). Experimental results show that group information improves arousal and valence classification, across all datasets, except for K-EmoCon on valence. The proposed method surpasses previous work results for K-EmoCon on arousal, and provides a new baseline on AMIGOS for long-videos. See more.