This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
UNDOCUMENTEDFindGuildByTag
https://api.steampowered.com/IDOTA2Guild_247040/FindGuildByTag/v1/
UNDOCUMENTEDGetGuildPersonaInfo
https://api.steampowered.com/IDOTA2Guild_247040/GetGuildPersonaInfo/v1/
UNDOCUMENTEDGetGuildSummary
https://api.steampowered.com/IDOTA2Guild_247040/GetGuildSummary/v1/
UNDOCUMENTEDSearchForOpenGuilds
https://api.steampowered.com/IDOTA2Guild_247040/SearchForOpenGuilds/v1/