There are a couple of ways how you can assign clients to the users.
Assign multiple clients to a user
You can edit the user profile and assign the client. If you just want to assign clients to one user, this is the best way to assign them.
Assign Multiple clients to multiple users
If you want to assign multiple clients to multiple users, this option is very useful.
Super Admin and Admin users don't need to be assigned the clients. They already have access to all the clients.