[Issa]

If you want to display the group number for every raid member, you can use this code:

if not UnitInRaid("PLAYER") then return end
for i = 1 , GetNumRaidMembers() do
    name, _, subgroup = GetRaidRosterInfo(i)
    if (name == Name(unit)) then
        return subgroup
    end
end

combined with the created event PARTY_MEMBERS_CHANGED that mrruben5 mentioned.