I'm going to ask my friends to reimburse me for the travel costs even though I didn't tell them that they'd have to pay.
Is there a reason why you want them to reimburse you?
I can't afford to pay for it alone.
If you invite people on a trip you should explain the costs at the beginning. You can't really expect people to pay you back if they don't have any money and thought it was a gift.
I didn't know if my friends were bringing money with them since it had been a long time since we communicated.
Yeah, perhaps that's on you too, it seems like you should have been the one to communicate with everyone prior to the trip. Asking them for money, might upset them and they won't go on a trip with you again. Just tell them what's going on and hopefully you can figure it out together.