This probably won't be added. It's been a very long time since new features were added to PokeBase, and if Pokemaster were to do a sprint for old times sake, there'd be some more pressing things he'd focus on first.
But it's not hugely important, as you can easily find users through one of the following methods:
Method 1
Here are the links to a bunch of user's profiles.
https://pokemondb.net/pokebase/user/Mewderator
https://pokemondb.net/pokebase/user/trachy
https://pokemondb.net/pokebase/user/DarkTyphlosion
You will notice the only difference is their exact username at the very end. If you delete that part and write in someone else's username, the link will work.
If the user you want to go to has special characters in their name, you can use URL encoding to negotiate that (or a different method if you don't know what that means).
Method 2
Do a Google search for the username, with the text site:pokemondb.net/pokebase/user
added to the search. This makes Google only show you PokemonDB users in the results (and random wallpost pages it has indexed, but those also work).
Here is an example. If it doesn't work, you can use site:pokemondb.net/pokebase
(without the "user" part) to see if it brings up a question they asked or similar.
Method 3
If you've gone to the user's profile before, it's probably in your browser history if you don't clear it often. You can look for the user there.