This is a tough question, as there is no easy answer.
The harsh reality is that people living with MS are often left out of social situations no matter how hard they try to be involved. I am currently in my senior year of high school. Midterms are next week, and to date, I have attended approximately 12 days of school. I am, however, able to go in for about an hour a day for my college class. My other classes are very seldom attended because of my increased fatigue. In the last four weeks, I have been battling a virus of some sort, which has heightened my symptoms. These classes are so seldom attended that some of my classmates don't even know I am taking the class Because of this, I have not gotten to know many people in my class.
As a high schooler, it is easy to feel left out. Friends form their own groups, and often become so closely knit, that adding others is a difficult task. So, when students like me who are seldom at school try to make strong friendships, it can be very difficult, and it can also feel quite lonely.
This feeling of being alone is not only for me as a student. This feeling affects people of all ages. A couple of months ago, while visiting with a fellow MS Society volunteer, I learned that my friend was facing the same challenges as me, but he is about 40 years older than me. This man was once very successful in his work, and had a high position at his place of employment. His close friends were all coworkers. His social life was based around his job; outings with co-workers, business meetings, and time in the office. However, 13 years ago he relapsed. He had to face the difficult decision to go on medical leave. Since then, he has lost many of his social outing opportunities. He is no longer working, but the people he used to spend time with, are. Invites never came or were never responded to. He felt, and feels, alone.
As I said before, there is no easy answer to not feeling left out of social situations. Those who do not understand how to react, or when to invite, simply do not extend invitations. People get left out. That said, there are ways to fight to stay in the "loop" of social outings. Although, I have found it takes persistence!
To stay connected with my friends, I continue to get involved. Volunteering, spending time at church, school activities, community events, or just inviting a friend to dinner. Just like any friendship, it takes work. The more you work to be involved in social situations, the easier it will get, and the more people will want to invite you. So, the long answer to this question, is that people living with chronic illnesses are fighters. We fight to advocate for ourselves, to live our lives day to day, to "fit in" to social gatherings, and to keep a positive outlook on life. No, it isn't easy. But we must continue to fight for ourselves.