Is there a way to tell the last time someone activated a group tag?

[Rolig Loon]

No, there isn't. Activating a group tag doesn't send any kind of a message to the system.  The only thing you could do is to write scripts that detect when a group member has done something like clicking on an object or entering a group-owned building.  If you kept track of the name (or UUID) of each person the script detected, and when, you'd know that the person at least had the tag on at that point.

[Rolig Loon]

No, there isn't. Activating a group tag doesn't send any kind of a message to the system.  The only thing you could do is to write scripts that detect when a group member has done something like clicking on an object or entering a group-owned building.  If you kept track of the name (or UUID) of each person the script detected, and when, you'd know that the person at least had the tag on at that point.