The server does not provide this information directly, but there are a few workarounds:
1. Check WoWProgress
If your guild is tracked on wowprogress.com, you can often find rough join dates there.
2. Use the In-Game Command (Yes, this now works in Classic builds)
In Retail, if players joined after the 2018 (BFA 8.0) Communities update, the server has their join date logged. This logging was not created server-side until then, so if you are in a guild prior to the 8.0 Patch, then the date will not be accurate. Players can run this macro themselves and report the date to you:
/run local t=date("*t",math.floor((tonumber(string.format("%.0f",C_Club.GetClubInfo(C_Club.GetGuildClubId()).joinTime))/1000000)+0.5));print("Date Joined: "..t.year.."-"..t.month.."-"..t.day);
3. Mass Set Join Date (Last Resort)
If you simply want to fill the blanks with a placeholder date (e.g., Aug 25, 2023) for everyone missing a date:
/run GRM_API.SetAllUnknownJoinDates(25,8,2023)