You can use chat messages via LokiPoe.Type to invite people to a party, but you cannot do any other party functions though the API like accept requests or change party settings, etc...
The reason the Party API was dropped for the new version was due to the change in the way the API has to work now. We no longer call client functions to perform actions directly, so all actions have to be performed using mouse/keyboard events and interacting with the GUI itself. The Social panel is a real mess in terms of trying to work with, so there's no planned support for parties under the current client setup. Much like for the same reason why assigning passive skill points isn't possible due to the complexity of the GUI.