Galaxy groups and clusters are the main tools to test cosmological models and to study the environmental effect of galaxy formation. This work aims to provide a catalogue of galaxy groups/clusters and potentially merging systems based on the SDSS main galaxy survey. We identify galaxy groups and clusters using the modified friends-of-friends (FoF) group finder that is designed specifically for flux-limited galaxy surveys. FoF group membership is refined by multimodality analysis to find subgroups and by using the group virial radius and escape velocity to expose unbound galaxies. We look for merging systems by comparing distances between group centres with group radii. The analysis results in a catalogue of 88 662 galaxy groups with at least two members. Among them are 6873 systems with at least 6 members which we consider as more reliable groups. We find 498 group mergers with up to six groups.