[Kanegasi]

Any nameplate you see has a valid unit in the form of nameplaten where n goes up to 40. All functions that accept a unit work with nameplate units.

As for looking for nearby guildies, in my experience, it's better to maintain a local table of guildies and then compare players to that list. This bypasses the awkwardness of GetGuildInfo (which isn't reliable outside of inspect distance) as well as avoiding querying the guild roster each time.

local guildie,myrealm={}
local function normalize(name,realm)
    realm=lower((gsub(realm or (match(name,"%-(.*)")) or 'nil','[%p%s]','')))
    name=lower((gsub((match(name,"(.-)%-")) or name,'[%p%s]','')))
    return realm~='nil' and name..'-'..realm or name..'-'..myrealm
end
local f=CreateFrame("frame")
f:RegisterEvent("GUILD_ROSTER_UPDATE")
f:RegisterEvent("NAME_PLATE_UNIT_ADDED")
f:RegisterEvent("PLAYER_LOGIN")
f:SetScript("OnEvent",function(self,event,unit)
    if event=="GUILD_ROSTER_UPDATE" then
        self.tguild,self.online=GetNumGuildMembers()
        if self.oguild~=self.online then
            self.oguild=self.online
            wipe(guildie)
            for i=1,self.tguild do
                local name,_,_,_,_,_,_,_,online=GetGuildRosterInfo(i)
                if name and online then
                    -- guilds on a connected realm list names with -realm in them
                    guildie[normalize(name)]=true
                end
                if #guildie==self.oguild then
                    return
                end
            end
        end
    elseif event=="NAME_PLATE_UNIT_ADDED" then
 
        if guildie[normalize(UnitFullName(unit))] then
            -- here's where the check happens
            -- you can either do something with the nameplate
            -- using C_NamePlate.GetNamePlateForUnit(unit) to get the frame
            -- or something simple like play a sound to announce the guildie
        end
 
    elseif event=="PLAYER_LOGIN" then
        local _,realm=UnitFullName("player")
        myrealm=lower((gsub(realm,"[%p%s]","")))
    end
end)

EDIT: I forgot that GetGuildRosterInfo returns a GUID. Also, the code above is basically a modified version of my own code in an addon that deals with name-realm data coming from sources that don't include a GUID and only deals with those online. Below is a much simpler version of caching your guild.

local guildie={}
local f=CreateFrame("frame")
f:RegisterEvent("GUILD_ROSTER_UPDATE")
f:RegisterEvent("NAME_PLATE_UNIT_ADDED")
f:SetScript("OnEvent",function(self,event,unit)
    if event=="GUILD_ROSTER_UPDATE" then
        local total=GetNumGuildMembers()
        wipe(guildie)
        for i=1,total do
            local guid=select(17,GetGuildRosterInfo(i))
            if guid then
                guildie[guid]=true
            end
        end
    elseif event=="NAME_PLATE_UNIT_ADDED" then
 
        if guildie[UnitGUID(unit))] then
            -- here's where the check happens
            -- you can either do something with the nameplate
            -- using C_NamePlate.GetNamePlateForUnit(unit) to get the frame
            -- or something simple like play a sound to announce the guildie
        end
 
    end
end)