Most people get put into the Work Related Activity Group (otherwise known as WRAG) - probably at least 80 or 85%. It means you are deemed to be able to engage in 'work related activity' i.e. talking to an employment adviser about your future return to the workplace, and what support you might need to arrive at that stage. The first appointment is with an Employment Adviser in the Jobcentre, the rest of them will be with someone in a private sector company contracted to the DWP to run the Work Programme.
The good bit: they can offer wellness classes such as walking groups, CBT, help with CV preparation, training to change occupation - all sorts of things. It does not involve being told to apply for jobs. They cannot ask you to do anything that would make your condition worse. There is really nothing to be afraid of. If they push you in a direction you are not happy with, complain to their manager and complain to the Jobcentre.
The bad bit: they can defer appointments if you are not well, but if you don't turn up without telling them, they will sanction your benefit. They can't take away your basic amount, but they can remove the top-up you received since you got put into WRAG.
Another major implication of being put into WRAG is that if you receive ESA as a contribution-based benefit (i.e. you have paid all your NI contributions because you have worked recently) you can only get ESA for 365 days. The only way it can continue after that is if you are also eligible for the means-tested version. If you have a partner who works, you are unlikely to qualify (unless they have an extremely low income).
The other group of people on ESA will have been placed in the Support Group (a misnomer in my view, as they don't get any 'support') because they are deemed too ill/disabled to participate in any work related activity. They get slightly more money, and the payments can continue for as long as they qualify. They don't get asked to any interviews usually, apart from people occasionally checking that they still qualify or asking if they want to volunteer for things. Just say no.
If there is even a vague possibility that you have been put into the wrong group, I would suggest you appeal the decision. Get help from an advice agency to do this. You can only win such an appeal if you can score the relevant points or the 'exceptional circumstances' rules apply. Check this document for the points relevant to the Support Group on pages 24-26: direct.gov.uk/prod_consum_d...
For 'exceptional circumstances' to apply, you need to show that there would be a substantial risk to your physical or mental health if you were found fit for work related activity. You need good medical evidence for any appeal, and it needs to be specific to the grounds of your appeal, not just saying you have Lupus. It is not about your condition(s), but about the functional difficulties you have because of the condition(s).
You generally have a calendar month to lodge an appeal once you have been given a decision, so don't delay telling them that you want to appeal and why, even if you can't provide the necessary medical evidence at that time. But do be aware that it is 99.9% unlikely that they will change the decision unless you can provide new evidence, so do try to get it to them as quickly as you can. As well as medical evidence it is ok to provide letters of support from carers/relatives if they can explain your functional difficulties because they have witnessed them.