As of now, Luminate Online does not record timestamps for when constituents are added to a group. Because of this, there is no way to identify who was in a group at particular points of time in the past. 

To know how many constituents are currently in a group:
  1. Select Constituent360 > Groups
  2. Search for the group name
  3. Click Edit under the Actions column
  4. The Group Summary section will display the number of users in the group
  5. Click Refresh Summary to update the Group Summary section