Add multi-agent papers on social influence and group effects

Hi, thank you for maintaining this excellent repository on multi-agent research! This PR adds our recent papers on human interaction with multi-agent systems: 1. (CHI 25 Poster) Greater Than the Sum of Its Parts: Exploring Social Influence of Multi-Agents https://doi.org/10.1145/3706599.3719973 2. (CSCW 25) Multi-Agents Are Social Groups: Investigating Social Influence of Multiple Agents in Human–Agent Interactions https://dl.acm.org/doi/pdf/10.1145/375763 Both papers examine how groups of LLM-based agents can function as perceived social groups and influence human judgments, social pressure, and opinion shifts. Given their empirical insights into social influence dynamics within agent societies, I believe these contributions fit well under the *Social Simulation & Agent Societies* category. (I'm not entirely sure whether I should update the `.bib` file, but I'm happy to add it if needed.) Again, thank you again for maintaining the sources. It's very helpful :D