Auto add multiple data points to a single cell

I am building a portal system and want to collate all associated users to a single cell.

The table i want to populate is a table for the building information, within the table i have multiple account connections for each of the companies users. When i allocate the companies to the buildings i want it to automatically add that user to a new cell that is just called Accounts.

At the moment i have tried Record Action and “Update this record” but it only adds 1 of the accounts, ive also tried setting up a task and that didnt work either… does anyone know how i can automate this task? thanks