As discussed this question Is lacking information. E.g. where does the information about the users come from and how to maintain it.

Please see this information for your education  https://jazz.net/forum/questions/203755/how-should-i-ask-a-question-in-the-forum-if-i-want-to-receive-useful-answers   

The following information can help you to understand the limitations of RTC better: https://rsjazz.wordpress.com/2016/07/15/rtc-process-customization-what-you-can-and-cannot-do/

This workshop (the RTC version) https://jazz.net/library/article/1093 provides you with a good overview what customization options are available.

To manage a list of users one approach could be the HTTP Filtered Value set. A list is published as XML and you can filter the data. It might be possible to select the user ID from a value set and store the information in a string attribute. There are limitations in what can be achieved.

I have created an example once that used the filtered value set and stored the data in a string attribute. A HTTP filtered value set returns one value that can be stored e.g.
User1 Name | Mobile | E-Mail | ....
User2 Name | Mobile | E-Mail | ....

I stored the whole customer information in that attribute in a form that I could use script based calculated values that extracted the information for display in other attributes. E.g I stored

User1 Name | Mobile | E-Mail | ....

And used the | to extract the information for display in the calculated values.