I'm still not back to normal after 3.5 years - actually I still feel quite limited despite having recovered a significant amount.

You just never know with these things.

I recommend that you let your co-chair or assistant chair or the board of the fund raiser know right away that you may need to back off the project or take on a much less involved role. if you get better before the fund raiser then you can increase the help you give, but backing off now should give them time to adjust just in case you're not all better by then.

Let your friends know that you are recovering from a concussion. Most likely, you should be well enough to have a good time with them when they are here. But there is a chance that you will need to take it easy.

There are many work arounds you could use when they are here if you do end up needing to take it easy. But I think if you can avoid worrying about it for now, since it's so far off, then that might be best? Or you could consider postponing their visit?