Pipeline formula for user names

I need a pipeline to populate a field in a new record with the names from another field. Many users did not update their profile with a ‘Screen Name’ so I can’t use that field in the pipeline. I am having trouble getting it to populate using the First and Last Names. What formula can I use in Pipelines to create a list of multiple names?

I used this field to create a text field and this is what is returned:               

Using this text field is not ideal as Barb does not have a screen name so it defaults to her email address. If there are several users, this field will be harder to read with emails being used.             

I want the pipeline to take the Fundraisers Assigned values and populate like this – preferably comma separated but semi-colon is fine too.

                Example: Amy Gosz, Barbara Burns

I can get the First Name and Last Name to combine but then it only returns one of the names. How do I write the formula to not only combine the First Name and Last Name, but also list the multiple users names? I need all names (can be several) that appear in the Fundraisers Assigned field to populate to the new field.