KSK membership is not compulsory but if required the application needs to be submitted direct to them and may take a few months - up to 8 months is not unusual - to be processed. Not only does one have to wait a couple of months but there is also no guarantee that every application is approved due to their strict adherence to the requirements.
The lengthy period having to wait for KSK's acceptance or possible non-approval of one's application, invariably produces concerns to those who in the mean-time might become in need of a residence permit. Although in certain cases extensions are granted, this is indeed very much up to the Foreigners' Office (Ausländerbehörde) poilcy, or even, the individual case-handler.
Another aspect people tend to forget is that after months of waiting, when the day finally arrives in which they receive KSK's acceptance letter, they are required to make back-payments on contributions towards the state pension. The amount due for 2019 is 9.3% of the assessable income, for both the "employee" (KSK member) and "employer "(KSK), totalling at 18.6%.
Thus, having waited e.g. 8 months on 12,000€ assessable income, 744€ (8 months x 93€) needs to be paid in full and within 2 weeks of receiving KSK's Certificate of Membership.
As a member of KSK this means that you are treated as an 'employee' rather than a freelancer or self-employed. As such you are entitled to join the statutory (public) health insurance (gesetzliche Krankenversicherung (GKV), which can be provided by more than 100 providers (Krankenkassen). Having secured this kind of health care, medical and long-term-care insurance are in place and the costs are equally shared between member and KSK.
The cost of German statutory insurance is related to income and you will need to provide KSK annual assessments of the anticipated annual income in form of a Profit & Loss Report and/ or Tax Return, supported by other acceptable documentation like: contracts, business proposals, orders, etc. The current rate for any GKV plan is dectated by legislation and for 2019 set at 14.6% of the individual's gross earnings. On top of this the the insurers charge a fee set at their own discretion, in average 1%. Lucikly the choice of GKV provider, as also the application procedure, is totally up to the member. Thus, one can shop-around for the best solution (monetary or benefically), providing KSK with a temporary membership certiciate when applying for KSK membership.