I’m pretty new to using DnD Beyond, is there a way for me to assign characters that I’ve created for the players in my campaign to my campaign in DnD beyond? Do they have to have a subscription to use it for their player management?

Just add the character to the campaign unassigned. Then when the player joins they can claim it.

Do they have to have a subscription to use them?

Quote from KelpieRunner >>

Do they have to have a subscription to use them?

 No, they don't need a subscription to use the characters.  Just a free account is fine enough to access the character sheet online.

Fantastic! Thank you!!