Description
After going through the current implementation of MultiSelect ,it seems that the component does not rendering custom list item and selected items.We can update it to accept renderItem and renderSelected props like the AutoComplete component.

The usecase for the same is mentioned here:
RocketChat/Rocket.Chat#24115

We can update the Add user menu to look somewhat like this:

Insert an image/gif
The current implementation in the Add user section is based on the AutoComplete component and it is certainly not a very good UI experience (More on it in the issue above) and has a lot of room for improvement.