1. Existing customers (i.e. customers who existed prior to setting up the Sign Up offer) will not receive Sign Up Offers. Only new customers created after the SIgn Up Offer was created and those who opt in to marketing will receive the offer.
2. Customers imported via CSV import from under Customers->Enrolled Customers will also not receive the Sign Up Offers. If they have opted in to marketing they will receive all other offers that they qualify for.
3. To receive signup offer a customer needs to have the marketing flag set at the first time they register in the system. If a customer enrolled without the marketing flag and later switch the marketing flag on, system still assume it as an existing customer and do not trigger signup offer. However these customers will receive all other offers that they qualify for.