Authors CurseForge

Well for one thing, you declare a function then fail to call it. You need to wrap the top level function in parens and invoke it, ie (function() ... end)()
You also need to add a return value and use that to set args (instead of the embedding it in an array then handing it to args).

Yes you want something like:

args = -- note no array braces
      (function() 
                           local result = {}
                           result[1] = ... -- fill the array
                           return result
      end)() -- call the function

It's also worth noting this function is called very early in startup when you construct the options array (before calling AceConfig). If you want code to run later (when the user opens the options window) you'll need to use one of AceConfig's entries that supports a function type.

I'm not sure why you are embedding a multiselect option with the name "rank" (where that's the actual name of a guild rank) inside a group option with the same name "rank"... if there are no other options in the "rank" group, just make the top-level "rank" option a multiselect instead of a group.

Anyway, I think you're actually trying to do something more like this:

-- First, define your options table, leaving the [i]args[/i] node as an empty table:
guildMembersGroup = {
	type = "group",
	name = "",
	childGroups = "tree",
	args = {},
}

-- Then, add a child node to the [i]args[/i] table for each guild rank:
for i = 1, GuildControlGetNumRanks() do
	local rank = GuildControlGetRankName(i)

	-- Define the individual option:
	local option = {
		order = i,
		name = rank,
		type = "multiselect",
		width = "full",
		values = function()
			local names = {}
			for j = 1, GetNumGuildMembers() do
				names[j] = GetGuildRosterInfo(j)
			end
			return names
		end,
	}

	-- Add it to the guildMembersGroup arguments:
	guildMembersGroup.args[rank] = option
end

In practice, you also need to define get and set functions so that your options actually do something when selected/checked/clicked/typed in/etc. However, since you did not explain what you actually want your GUI to do, and you did not post your actual entire code, I'm not even going to try to suggest what those functions should look like.

You would probably also want to update the options table in response to events that indicate changes in the guild structure (eg. you added, changed, or removed a rank) or roster (eg. someone joined the guild, got promoted or demoted, or left).

It's because it's creating a new table each time you click a rank to populate the option values (see my code above). If you're really concerned about garbage creation in an options menu, you can modify it to reuse the same table over and over for each rank:

-- Then, add a child node to the args table for each guild rank:
for i = 1, GuildControlGetNumRanks() do
	local rank = GuildControlGetRankName(i)

	[B]-- Define a reusable table:
	local names = {}[/B]

	-- Define the individual option:
	local option = {
		order = i,
		name = rank,
		type = "multiselect",
		width = "full",
		values = function()
			[B]-- Clear the reusable table's previous values:
			wipe(names)[/B]
			-- Fill the table with current values:
			for j = 1, GetNumGuildMembers() do
				names[j] = GetGuildRosterInfo(j)
			end
			return names
		end,
	}

	-- Add it to the guildMembersGroup arguments:
	guildMembersGroup.args[rank] = option
end

You also mentioned that you wanted each rank to only show the players who were currently at that rank. To do that, you'd want to modify the code some more:

			-- Fill the table with current values:
			for j = 1, GetNumGuildMembers() do
				-- Check if the player is at this rank:
				local name, _, rank = GetGuildRosterInfo(j)
				if rank + 1 == i then
					-- This player is at this rank, add them:
					names[j] = rank
				end
			end

Fixed. Also, the rank variable should be used in the next line instead of calling GetGuildRosterInfo again. ;)