Understand symptoms may come and go.
People with invisible illnesses often suffer from extreme fatigue and chronic pain. Sometimes just getting through the day is difficult, leaving them with no desire to go out socializing after work. On the flip side, don’t assume that because they have an illness that they won’t want to go out–they might be having a good day and are feeling great. Be inclusive but respect the person’s decision, only they will know whether they are up for the activity.