The low tech way is to use a List user field. You get to choose multiple users in one field. I assume that your current app is using a field of type "User" and not type "List User". You will have to create a new field.
If you have a lot of data I could explore for you easier ways to get the data across. It is possible that creating a list user formula field which has the value f the user field and then changing that to a List user data entry field will migrate the data across, but I have not tested that.