While common, it is far from best practice to have all users connect as a single login.
In fact, best practice involves using one or more "ownership" accounts to own institutional table resources (feature datasets), using individual user logins to access the geodatabase, and making use of roles to assign access to tables by users. Not only does this practice reduce contention to shared resources, it also helps identify who is acessing what, and when.
Note that the RDBMS implements the security model, not ArcSDE (there is no such thing as an "sde group"). This is just proper use of the RDBMS' own security model.