No, it is not necessary for each user to have a smartphone.
On the other hand, each user must have created an iHealth account and be added as a new user in the scale. See article How to add a new user.
Since the iHealth MyVitals app is multi-user, you can track multiple accounts from a single smartphone.
*credit to Sylvain of iHealth Labs EU
https://helpcenter.ihealthlabs.eu/hc/en-gb/articles/115005876245-iHealth-HS5-Does-each-user-have-to-have-a-smartphone-