We just added an often-requested features: to be able to charge new members a fee, either for a deposit or a signup fee.
Before you had to manually create a charge every time a new member signed up. This was of course not very convenient and also error-prone. What you can do now instead is configure an automatic charge on a per-plan basis.
To add the charge add or edit a plan and scroll down to the signup charge section in the basic settings tab. Enter an amount and a description — from now on new members choosing this plan will have a charge added to their account, which will in turn be added to their first invoice.