There is no specific command, but you can use this query:
Code:
select sr.name, sl.name
from syssrvroles sr, syslogins sl, sysloginroles slr
where sl.suid = slr.suid
and sr.srid = slr.srid
and sr.name like "sa_role"
In the last line, you can substitute
sa_role with any role-name you want to know the members of.