I have made some characters I intend to be NPCs for an upcoming campaign. Is there a way to add them as an unassigned character so I can include them in the Encounter Builder "Manage Characters" section, or just so it's easier to remember which ones are in use?

You can join a campaign with an existing, ready made character and then unassign it 

Quote from Davedamon >>

You can join a campaign with an existing, ready made character and then unassign it 

 But I am the DM for the campaign, I cannot see an option to add a character, only create one

Quote from trapbuilder2 >>

Quote from Davedamon >>

You can join a campaign with an existing, ready made character and then unassign it 

 But I am the DM for the campaign, I cannot see an option to add a character, only create one

You can use the invite link

Use the join link.

Thanks guys