Rumen microorganisms can effectively digest lignocellulosic biomass. Various rumen microbial groups (bacteria (B), fungi (F), and protozoa (P)) make different contributions to the fermentation of the biomass material. In this study, physical, chemical, and antibiotic methods were used to treat rumen fluid to obtain the following groups: whole rumen fluid (WRF), protozoa + bacteria (PB), fungi + bacteria (FB), fungi + protozoa (FP), bacteria (B), protozoa (P), fungi (F), and negative control (CON). Subsequently, a biogas slurry was added to each rumen microbial group (at ratios of 1:1, 3:1, 1:3, 5:1, and 1:5 v/v) and the mixtures were used to ferment switchgrass. These mixtures were labeled on the basis of their inoculum and ratio; for example, the WRF and biogas slurry at ratios of 1:1, 3:1, 1:3, 5:1, and 1:5 (v/v) were labeled as WRF1, WRF2, WRF3, WRF4, and WRF5, respectively. The efficiencies of digestion by all mixtures were compared to that of monodigestion with the biogas slurry (CK). The results showed that rumen bacteria play an important role in improving anaerobic digestion in the cofermentation system. The FB2, WRF3, FB5, and PB5 groups of cofermentation achieved methane production values of 189.5, 180.7, 176.9, and 174.5 mL/g of volatile solids (VS), respectively, which were all significantly higher than that of CK (137.9 mL/g VS), with increases of 37.37, 31, 28.24, and 26.5%, respectively. The degradation rates of total solids, glucans, and hemicellulose in CK were almost the same as or higher than those in the mixed inoculum groups, but CK achieved lower biogas and methane production than did FB2, WRF3, FB5, and PBS. Therefore, cofermentation can improve the efficiency of anaerobic digestion and enhance the efficiency of methane conversion.