Tribalism implies the possession of a strong cultural or ethnic
identity that separates one member of a group from the members of
another group. Based on strong relations of proximity and kinship,
members of a tribe tend to possess a strong feeling of identity.
Objectively, for a customary tribal society to form there needs to be
ongoing customary organization, enquiry and exchange. However, intense
feelings of common identity can lead people to feel tribally
Tribalism is alive and well in the modern world. Persons can act in ways that are opposed to their own economic self interest, or even in ways that defy common sense if they are motivated by the interests of the tribe where these persons derive some or all of their self-identity.
In your group which is becoming a cult, members increasingly define themselves or come to be defined by their membership in the group. It could help if there were a charismatic leader such that membership in the group was membership in his group. As the social connections with group members are more and more involved with the group and its leader and less with other entities in the outside world (family, church, jiu jitsu league) pressures from within the group will be more significant and members will be less willing or able to leave the group.
Important to note: this process does not mean the group has nefarious intent! This sort of thing can happen with any group, and cult-like influences might be very important for one individual and unimportant / unnoticed for another. It depends on the relationship of the individual with the group.