In wplms 2.9.2 we have added groups assign feature to wplms custom registration forms .
In custom registration form you will see a new settings for groups .
This select option is divided into two option categories :
i. Enable user to select from all groups
ii. Select selected groups to show in dropdown to select .
Note here that if you want that user should be added to only one group when registers through this registration form then select only one group from the list of group .There will be no dropdown shown to user but she will be added to that group upon registration .
