The 9th value from
GetGuildRosterInfo() should return whether or not they're online. Also the level cap is already 85, so you only need to check if they're 80+.
Code:
/run for i=1,GetNumGuildMembers(true) do local name,_,_,level,_,_,_,_,online=GetGuildRosterInfo(i); if online and level>=80 then InviteUnit(name); end