addFriendToGroup(groupID, userSteamID, callback) {
		return StdLib.Promises.timeoutCallbackPromise(10000, null, callback, true, (resolve, reject) => {
			let sid = Helpers.steamID(userSteamID);

			this._send(EMsg.AMClientAddFriendToGroup, {
				groupid: groupID,
				steamiduser: sid.getSteamID64()
			}, (body) => {
				if (body.eresult != EResult.OK) {
					return reject(Helpers.eresultError(body.eresult));
				}

				this.myFriendGroups[groupID].members.push(sid);

				return resolve();
			});
		});
	}
Add an user to friends group (tag)
@param {int} groupID - The friends group
@param {(SteamID|string)} userSteamID - The user to invite to the friends group with, as a SteamID object or a string which can parse into one
@param {function} [callback]
@return {Promise}