Add "Last Activity Date" to the Billable Members API

Problem to solve

There is no way to view the Last Activity Date of your group members on .com (either through the GUI or via the API)

Intended users

• Delaney (Development Team Lead)
• Sidney (Systems Administrator)
• Sam (Security Analyst)

Further details

Since we don't allow Admin access on .com, Group administrators need a good way to view the Last Activity Date of members in their group. Currently, you can sort users by Last Activity, but it doesn't show an actual date of that activity.

Proposal

• Add a Last Activity Date field to the Group Member page (being handled via #33457 (closed))
• Add Last Activity Date to the Group Member API

Permissions and Security

I believe the Last Activity Date is currently restricted to the Admin API, but it would make sense to add it to the Group Member API as well.

What does success look like, and how can we measure that?

If .com Group Owners are able to view their user's Last Activity Date.

What is the type of buyer?

This should be a Core feature. Everyone on .com should be able to effectively manage users in their Group.