A guildmaster is highly qualified technically and has guild master status but devotes more of the time to lead/manage/moderate a guild. The term dates from medieval times as does grandmaster, which describes someone who focuses on research.
 ECG use
Any ECG master for instance that volunteers to maintain ECG dogma is a guildmaster. There is no special ECG-defined role nor any special status associated with this role - ideally it would remain almost invisible and executed by anonymous trolls. It's possible also for several individuals to combine to act as one guildmaster, i.e. an ECG patroll can serve in this role as long as they have credentials, including blind credentials, showing all are ECG masters, and commit to a means to verify that they are not cooperating to gain any extra vote.
Any identified individual who mediates for the guildmasters is called a mandarin. A mandarin may actually be a guildmaster in their own right, or speak for a group of them.