Is it legal to add people to a Telegram group without their permission?
Telegram is build in such a way, that by default, one does not require to obtain an additional consent to invite someone to join a group or channel.
By default, users have enabled "open to invitations" settings in their profile, meaning they have granted the consent to such invitations. In case a user will turn that off, Freegram will not be able to invite such user to a group.
We do not provide any illegal services, Freegram is simply using one of the functions of Telegram app to bring awareness about your project to a wider Telegram audience.