Both the Grouping method and 10% method are similar. They differ in how they define the closely spaced mode band. The Grouping method presumes that modes that fall within a group are within a 10% band, while the 10% method considers modes within 10% of a mode of interest, that is, taking 10% above and below the mode of interest; an effective mode band of 20%.
In both cases, these closely spaced modes are absolutely summed within their group, based on the assumption that they will likely see their maximum response at the same time during the dynamic response. In both cases, modes that are outside of the prescribed closely spaced mode band are presumed to see their maximum at a different time, so are added to other modes and grouped mode summations using the SRSS method.