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.
UNDOCUMENTEDGetFriendList
https://api.steampowered.com/ISteamUserOAuth/GetFriendList/v1/
UNDOCUMENTEDGetGroupList
https://api.steampowered.com/ISteamUserOAuth/GetGroupList/v1/
GETGetTokenDetails
https://api.steampowered.com/ISteamUserOAuth/GetTokenDetails/v1/
NameValueTypeRequiredDescription
string Yes OAuth2 token for which to return details
UNDOCUMENTEDGetUserSummaries
https://api.steampowered.com/ISteamUserOAuth/GetUserSummaries/v1/