I believe it’s really important to have as much diversity as possible because only associating with people who are similar to you can potentially be de-motivating, and even demoralising.
For example, if you and all of your friends have 20lbs to lose, the chances are you will judge yourself against them. If one doesn’t do as well as you, this may very well boost your self-esteem (but it’s hardly being supportive to the other person); however, what if two or three of your group progress faster than you? How will that make you feel? Will it spur you on or will it make you think there’s something wrong with you, that you cannot be as ‘good’ as them?
I’m not saying it’s not advantageous to have friends in similar situations, it *is*; however, exclusivity is not always the best idea.
Having a diverse network means a diversity of methods. And that is a very good thing because we are all different, and our bodies work in different ways. Plus, we all have something to offer ...and to learn from others. What works really well for me, may not work for you. And vice-versa.
But you won’t know what works for you until you do something proactive.