addAuthorizedBorrowers(borrowersSteamID, callback) {
		return StdLib.Promises.timeoutCallbackPromise(5000, null, callback, true, (resolve, reject) => {
			if (!Array.isArray(borrowersSteamID)) {
				borrowersSteamID = [borrowersSteamID];
			}

			this._sendUnified('DeviceAuth.AddAuthorizedBorrowers#1', {
				steamid: this.steamID.getSteamID64(),
				steamid_borrower: borrowersSteamID.map(sid => Helpers.steamID(sid).getSteamID64())
			}, (body, hdr) => {
				let err = Helpers.eresultError(hdr.proto);
				return err ? reject(err) : resolve();
			});
		});
	}
Add new borrowers.
@param {SteamID[]|string[]|SteamID|string} borrowersSteamID
@param {function} [callback]
@returns {Promise}