Is it required to have your own PicDrop account as invited team member?

The short answer:

No, team members do not need their own PicDrop account!

The detailed answer:

If you send an invitation to a new team member, we will check if there already exists a PicDrop account under this e-mail address.
If yes, the new team member can simply use the existing PicDrop account and the new team membership to your PicDrop account simultaneously without having to log in and out.
If not, we will send the new team member a link to create an account which will only have access to your shared PicDrop account. We do not force your team members to create their own, independent PicDrop account.