Energy Communities (ECs) have been introduced as a solution to address energy system development from a multidisciplinary perspective, with energy-related, environmental, and social targets as main pillars. While some countries still struggle with implementing basic rules for ECs, others, such as Austria, already go one step further and address the topic of allowing to participate in multiple ECs at the same time. The scientific literature to date lacks of studies that investigate whether it is profitable for a new participant to join multiple ECs instead of joining only one, and, which effect this would have on the existing ECs. Thus, this paper introduces a mathematical framework to optimally allocate a new participant within an existing set of ECs. The proposed economic-based decision making problem is formulated as a Mixed Integer Linear Programming (MILP) problem. While the model determines the economically optimal solution for a new participant, it is ensured that the initial situation of the existing ECs does not worsen. Results are provided for a set of ECs with different availability of resources and located in different settlement patterns, applying current Austrian legislation. Result shows that having multiple possibilities to share energy surplus and to satisfy energy demand can result in additional revenues of 6%, making multiple participation a valuable option to be considered by policy makers.