In Group Tracker field

Since Tiki4

This field allows to display if the user of the item belongs to a group (will display Yes or No) or to display since when he is in the group

The parameter contains

the name of the group
eventually the string 'date'

Example:
Teachers
Teachers,date

To work the tracker must have a field of type user selector with option 1 (creator).

Note: As the date where a user is assigned into a group is a new feature in Tiki4, it is possible that for old assignment the date is not known.

Tiki >=6.1
For instance, you can use that field in a tracker, and then request with a PluginTrackerList to list all records from a user tracker which belong to a specific group, and show all their details: phone, address, etc.

See also: User Groups tracker field, which is more powerful.