Yeah, it would be a little more convoluted doing it that way. But if you have a shortish list you can export the explorer results as a CSV, open up in notepad, and copy&paste them into the Add Users box:
You might be able to do 250 in one go, though I haven’t tried with that many. A handful of batches should cover it though, if not.
It should also be possible through the API using a PUT request to the eg /groups/49/members.json endpoint (you’ll need your own group_id) with a list of your usernames/emails.