Welcome to the community, @Emily_Morgan! The “is any of” option is only available for single- or multiple-select fields because they have a known number of options. I can see the benefit of expanding this concept for other field types, though. You might consider adding a request in the #show-and-tell:product-suggestions category (perhaps after a search to see if a similar request already exists).
If the view you’re setting up will always isolate entries tied to that specific group of trainers, you could make a formula field to do the search, with the formula outputting a 1 if any of the trainers is found, and a 0 if not. Then you would add an option to your filter to show any records where that formula field returns a 1.
Here’s an example formula. Keep in mind that with a lookup field, the value seen by the formula might be an array under certain circumstances, which is why I am concatenating that with an empty string first before looking for a name:
FIND("Serena", Trainer & ""),
FIND("Joanna", Trainer & ""),
FIND("David", Trainer & "")