We can make a very strong argument for belonging as a theme in Lord of the Flies. Numerous characters express this theme, striving to belong, reacting to being slighted by the group, and changing in order to fit in.
The strongest character example of the theme of belonging is Jack. Jack is introduced as the leader of a group interested in demonstrating his position with the choir. Later, Jack breaks with the main group as a response to being slighted by Ralph.
He bristles when Ralph doesn’t respect his getting meat for the group, only talking about the fire.
The rebellion of savages which Jack leads can be interpreted as being entirely concerned with belonging. Jack feels he does not belong to the larger group led by Ralph and so invites other boys to join him. They do and in creating the new group create physical signs of affiliation, painting their faces and bodies.
Piggy is another character expressive of the theme of belonging. At the outset of the novel he is happy to have a chance to have a fresh start with a new social group. In telling Ralph about himself and about his name, he reveals the fact that he has been an outsider. On the island he has a chance to be part of the group.
Finally, Ralph is another example of the dangers of existing outside of the group. After Sam and Eric have joined Jack's group, Ralph is alone which leads him to be deemed an enemy and hunted down.
Ralph's lack of belonging, his outsider status, manifests the social risks seen earlier in the novel as a set of physical dangers.