Until a client organisation has its first Admin user, agency users with Admin access can invite new client users.
To invite a new client user, just go to the Clients tab and select the right client. Then click 'Invite user'.
When you've clicked 'Invite user', a window will pop up where you can fill in the user's email, their roles, and their preferred site.
When you've filled those out, just click 'Send invite' and an invitation to complete their profile on JoinedUp will be sent to the new user.
Once a client organisation has any user set up under it with the Administrator role, agency users will no longer be able to set up new client users. This must be done by the client's admin users.