Adapt GroupController endpoints to use group name instead of ID

[adamczykba]

Summary
Lately the getGroup endpoint that was using the name of the group instead of its id was introduced.
As it was stated that in case of our configuration, the name will always be unique, let's make the other enpoints to use groupName to keep the API consistency.

Details
N/A

Acceptance criteria

• GroupController#addMember is refactored
• GroupController#getMembers is refactored

Additional context
N/A