Sounds to me that he is really into you, as are his friends. It's a good thing, of course, but going on holiday may be a little bit awkward for you. If it's just you with eight men, it might be a little strange. Seems like he really wants you to go, and his friends must be alright with it if they are letting him invite you. At the end of the day, it's up to you what you do.
If you go, try and spend time with them as a group aswell as having time alone with your boyfriend. This will be a good chance for you to spend some time getting to know his friends more, and getting to know your boyfriend more.
If you don't want to go, see if you two could have a little break somewhere else. It could be for a week, two days, whatever. There is no harm in asking if you two could have some quality time together rather than going on this holiday.
I don't know what I would do, and i'm sure this post is useless but nobody can make the decision for you. Only you know if you want to go, and only you can make the decision. It has some good points, it has some bad but maybe the good points overrule the bad ones, it's all down to you.