No, it is not necessary to have a smartphone of each user of the scale.
The app iHealth MyVitals is a multi-user app, so you can manage several iHealth accounts from the same smartphone.
However, it is necessary that each user has an iHealth account and has been added as a new user to the scale. See the article How to add a new user.
*credit to Sylvain of iHealth Labs EU
https://helpcenter.ihealthlabs.eu/hc/en-gb/articles/207932169-iHealth-Core-HS6-Does-each-user-need-to-have-a-smartphone-