Most people are not the ones initiating a hangout with others who they aren't yet friends with, they usually join in when invited if it fits their schedule/they are bored, etc. I'd recommend trying to brush off a bit the feeling of rejection since you don't have these connections yet. It sucks a bit but knowing it's completely normal might help you not feel it's something personal against you.
My recommendation is to keep trying with the ones you felt you could potentially click, I also have ADHD but never had much of an issue to make friends, and never made it a "goal", I just kept getting interested in people and would try to meet them again, most times it fizzles out even though you can have a nice time together but eventually some people stick around and become friends over time :)
Also try to have a well defined event, and be genuine, do things you like and try to invite people you met with these shared interests, in a casual and friendly way, people are more prone to join in when they don't feel there's pressure to do it.
I think the 3 first points of this comment [0] touch very much on the core of it, if you can strengthen against the feeling of rejection, and learn to be open and vulnerable (while balancing it to not become oversharing) it can go a long way to make people see and connect with you. Almost everyone is "in the closet" somehow, putting up a mask, when people meet someone that shows less of a mask and more themselves they usually will find those interesting.
What are some social activities you like to do?
[0] https://news.ycombinator.com/item?id=46417195