Coalitions play a central role in achieving both individual gains and common goals, such as advancing sustainability and reducing CO₂ emissions. This study examined how reputation, operationalized via Social Value Orientation (SVO), influences trust and coalition behavior. Using a real-time negotiation experiment, we combined pre-survey data with interactive group decision-making in a simulated transport setting. Participants (N = 681; 227 triads) first completed an incentivized SVO measure two weeks prior, indicating their prosocial or individualistic orientation. They were then matched in triads to play an incentivized weighted majority game simulating coalition formation among transport companies. Coalitions required at least two players, who negotiated how to divide a monetary incentive. In the experimental condition, participants saw counterparts' SVO scores via a simplified gauge as a reputation signal; the control condition included only company information. Trust was measured with a two-dimensional scale we developed, capturing predictability (reliability) and benevolence (prosociality), assessed at three stages (pre, post-manipulation, post-negotiation). Contrary to expectations, not all players were equally affected by the reputation manipulation. Player A, the strongest actor, was more often included in coalitions, contributing to a significantly higher frequency of grand coalitions (ABC) when reputation was visible, attenuating the classic Strength-is-Weakness effect. Player A was also perceived as more benevolent at the second trust measurement, when reputation was revealed. Exploratory analyses showed that prosocial participants were more often included, especially when their reputation was visible. They proposed less self-serving offers, formed more inclusive coalitions, and earned more. The unexpectedly high proportion of prosocials (78.6%) likely amplified trust and collaboration under visible reputation. Exploratory analyses further showed that trust—specifically predictability—shaped coalition inclusion and payoffs. Taken together, the results show how early social information shapes trust and negotiation in multi-party decision-making, with implications for organizational collaboration and sustainable resource-sharing.