How to perform burden test using plink /seq

Question

[aiswaryabioinfo]

I am trying to run burden test using Plink/Seq tool. I have a set of control and diseased vcf files for rare disease. I am getting a message like this while attempting burden test in Plink/Seq

pseq Aparna_BT assoc --tests burden --phenotype phenotype
LOCUS   POS     ALIAS   NVAR    TEST    P       I       DESC
pseq error : no grouping mask specified with a group command

Can anyone guide me how to use group masking feature