Create groups endpoint

[MartinBenediktBusch]

Overview

Create groups endpint that shows all usersToGroups relationships for a given group id: groups/[id]/users-to-groups

Expected Output

groupId: 'aad04a6b-7759-435a-bdab-3db7c44496f2',
      users: [
        {
       id: xxx
      username: xxx,
      firstName: xxx,
       lastName: xxx
    },
         {
       id: xxx
      username: xxx,
      firstName: xxx,
       lastName: xxx
    },
      ]
    }