Multiple users for the same service/bank account:
If you and your family members or business partners share e-services, you need to contact the service provider and give additional users a mandate/authorization (or a power of attorney, depending on the service) to access that account.
For example if you have a common bank account, you need to visit your local bank office where you can register additional users, set their limits etc. Each user will have their own access to the bank account.
Multiple users for the same Smart-ID account:
Like an ID-card, mobile-ID or passport, the Smart-ID account should only be used by the person it was authorized to, as that person is legally responsible for all the actions taken with that account. If you share your device with someone, do not share your Smart-ID PIN-codes with them!