In a psychosocial treatment study, knowing which participants are likely to put forth adequate effort to maximize their treatment, such as attending group sessions and completing homework assignments, and knowing which participants need additional motivation before engagement in treatment is a crucial component to treatment success. This study examined the ability of the Repeatable Battery for Assessment of Neuropsychological Status (RBANS) Effort Index (EI), a newly developed measure of suboptimal effort that is embedded within the RBANS, to predict group attendance in a sample of 128 middle-aged and older adults with schizophrenia. This study was the first to evaluate the EI with a schizophrenia sample. While the EI literature recommends a cutoff score of >3 to be considered indicative of poor effort, a cutoff of >4 was identified as the optimal cutoff for this sample. Receiver Operating Characteristics curve analyses were conducted to determine if the EI could predict participants who had high versus low attendance. Results indicated that the EI was successfully able to discriminate between group attendance, and this measure of effort appears to be most valuable as a tool to identify participants who will have high attendance. Of interest, overall cognitive functioning and symptoms of psychopathology were not predictive of group attendance.