the group which is able to communicate with at least a communication device of another individual of the group fall behind means that at least two individuals of the group are unable to connect which is defined as a fall behind event connectivity means the capability of connecting to the network formed by the group the disclosed embodiments analyzes the location relationships and network topology for the fall behind individuals and explores the characteristic that only individuals on the outskirt part of the group activity called outside have the possibility to fall behind accordingly the disclosed embodiments of the present invention continuously monitor the individuals on the outside of the group to determine whether a fall behind event has occurred fig2 shows an exemplary schematic view illustrating the location relationship of the network topology of a fall behind individual consistent with certain disclosed embodiments of the present invention as shown in fig2 network topology 200 includes 9 individuals and with individual 0 1 2 3 4 5 as possible fall behind candidates while individuals 6 7 8 as impossible to fall behind in other words only individuals on the outside of the group i e individuals 0 1 2 3 4 5 have the possibility to fall behind hence the disclosed embodiments of the present invention also disclose an outside determination algorithm to determine the individuals on the outside part of the group the outside determination algorithm will be described in details later the present invention is applicable to a group activity environment fig3 shows an exemplary system topology of the automatic fall behind warning system applied to.