Can you please share the way you tried to implement it?
I tried to find the relationship group entity here but there is not.
Dear Stefano Bassoli,
Thank you for the clarification.
In this case, we would recommend you creating a new [Lookup] in the [Lookup] section based on the "Relationship entity in group" Object.
After creating this object you would need to set it up.
Navigate to the [View] -> Select filed to display -> [+] button.
In the popped up minipage you would need to specify the Entity group name:
This will ensure that the Entity Group name would show up (In this example I have created two groups named Entity group 1 and Entity group 2)
Please be advised that there is no way of pulling up the [Contact]`s or [Account]`s names as they would be considered as Entities by this Lookup and would have only an ID available in the "List setup":
The Final result should look like this: the Entity Group name and the Record`s Id assigned to that Entity Group