Each client has 1 client portal login as explained in this article. This cannot be changed and we'd typically suggest the client just shares their login with those on their end if applicable.
Alternatively, client groups can have multiple users linked to them. To do this:
Create client group and put client into that client group
Create relevant customer users.
Link those users to the original client group created.
Those users can now see the details of the properties linked to the clients.