This is the 12530th most frequent Polish word.
należącej
"Belonging"
Here, 'należącej' is associated with the community that is part of the ecological group, showing its membership in the group.
Społeczność należącej do grupy ekologicznej zorganizowała marsz.
The community belonging to the ecological group organized a march.
The word 'należącej' here functions as a part of a participial phrase modifying 'książka' and indicates possession by Maria.
Książka należącej do Marii została zgubiona.
The book belonging to Maria was lost.
In this sentence, 'należącej' is describing a team member being part of the team, indicating association.