Can I determine an effective length factor β or kcr for sets of members in RSBUCK?
You can only use the results of individual members. As the governing member for the set of members, you can assume the one with the smallest critical buckling load Ncr.
Alternatively, it also helps to look at the axial forces in the individual members. If they are constant over the set of members (with a constant cross-section), then the effective length factors are also constant, so that you can use them also for the set of members.