No, it is not necessary for each user to have a smartphone.
However, each user must have their own iHealth account.
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/360000007749-iHealth-Lite-HS4S-Does-each-user-have-to-have-a-smartphone-