Low Earth Orbit (LEO) multibeam satellites will be widely used in the next generation of satellite communication systems

whose inter-beam interference will inevitably limit the performance of the whole system. Nonlinear precoding such as Tomlinson-Harashima precoding (THP) algorithm has been proved to be a promising technology to solve this problem

which has smaller noise amplification effect compared with linear precoding. However

the similarity of different user channels (defined as channel correlation) will degrade the performance of THP algorithm. In this paper

we qualitatively analyze the inter-beam interference in the whole process of LEO satellite over a specific coverage area

and the impact of channel correlation on Signal-to-Noise Ratio (SNR) of receivers when THP is applied. One user grouping algorithm is proposed based on the analysis of channel correlation

which could decrease the number of users with high channel correlation in each precoding group

thus improve the performance of THP. Furthermore

our algorithm is designed under the premise of co-frequency deployment and orthogonal frequency division multiplexing (OFDM)

which leads to more users under severe inter-beam interference compared to the existing research on geostationary orbit satellites broadcasting systems. Simulation results show that the proposed user grouping algorithm possesses higher channel capacity and better bit error rate(BER) performance in high SNR conditions relative to existing works.