This could mostly be done with a template change, the API will already return the following data (pasted at the end of this post), so you could add certain info already, like user Postcount and Reputation directly to the topic.tpl. You could then probably get the api data for each user in a topic and use "joindate"
to display when they signed up.
"user": {
"uid": "970",
"username": "a_5mith",
"userslug": "a_5mith",
"reputation": "779",
"postcount": "2168",
"picture": "https://i.imgur.com/Nf3xsUg.jpg",
"signature": "Redacted for ease of formatting",
"banned": false,
"status": "online",
"groups": [
{
"name": "3rd Party Developers",
"hidden": "0",
"userTitle": "Plugin/Theme Dev",
"icon": "fa-cogs",
"labelColor": "#2b8000"
},
{
"name": "Community Representatives",
"hidden": "0",
"userTitle": "Community Rep",
"icon": "fa-bullhorn",
"labelColor": "orange"
}
],
"custom_profile_info": []
},