Hi there,
If it is a simple project, just make a field where clients could be typed/picked up, then upon adding/updating validate that these clients actually exist and then extract their Ids and store them comma separated inside the User record. This way you avoid extra association table.
Then when you access the data, extract those Ids from a user record, add them to EF query, provided that those data records somehow associated with the client (having client Id directly or via some relation).
I've done it this way multiple times, should you be interested I can show you the code privately.
Regards, Sergii