Hi,
you can do this with simple conditions:
- Create a Select field with these options
groupA|Group A
groupB|Group B
groupC|Group C
- In this fields go to tab "Parameters" and set these conditions
1. if equal to Custom Value "groupA" add to usergroup Group A
2. if equal to Custom Value "groupB" add to usergroup Group B
3. if equal to Custom Value "groupC" add to usergroup Group C
(see screenshot for example)

NOTE
Conditions have 2 ways, so if condition if not met then it remove from usergroup, for example:
if user choose Group B:
1. Remove from usergroup Group A
2. Add to usergroup Group B
3. Remove from usergroup Group C