[Phanx]

You should avoid using select(). Function calls are basically the slowest thing you can do, and are totally unnecessary here. Plus, you're actually using two calls to GetGuildRosterInfo() instead of just using variables properly.

Here is some code that will keep track of who is currently mobile in your guild:

local mobileStatus = {}

local eventFrame = CreateFrame("Frame")
eventFrame:RegisterEvent("GUILD_ROSTER_UPDATE")
eventFrame:SetScript("OnEvent", function()
	wipe(mobileStatus)
	
	local _, numOnline = GetNumGuildMembers()
	for i = 1, numOnline do
		local name, _, _, _, _, _, _, _, _, _, _, _, _, isMobile = GetGuildRosterInfo(i)
		mobileStatus[name] = isMobile
	end
end)

Then, when you want to find out if a specific player is mobile, just check their status in the table:

if mobileStatus[name] then
	print(name, "is using the Mobile Armory app.")
else
	print(name, "is logged into WoW.")
end