When adding a member to the system, you have the ability to purchase a membership right from this screen:
If you prorate the first amount due on a membership, the prorate will also apply when memberships are purchased through this screen. Prorate settings are turned on under Tools>>System Settings>>Membership Settings. In order to prorate from the add user screen, a specific billing day must be set for memberships. For example, you can have your memberships default to the 1st, 5th, or 15th. If you do not have a billing day set or use multiple billing days, the membership prorate will not work and the billing day will default to the purchase day when purchasing from Add User.