this covers a range of activity from tying your wrists to the bedposts with silk scarves, to rather horrible things with leather or wet rope... So it depends. Would it turn you on? Then enjoy. If you don't think it would particularly, but you enjoy turning him on, same advice. But if you think you're going to hate it... well, I suppose you could try it just the once in case you're wrong; or you can just say no. But don't do anything you're uncomfortable with; that's not what sex is for.
Hmmm, i suppose its summat you both have to agree on. Do you not like the other girl or something. Maybe is afraid of what you would do to the other girl if you got loose. Is that why he has to tie you up? Really though, you should just stick to sex between yourselves and tie each other up...
I think for the most part sexually most people should try most things once to see whether they like things or not. With that in mind, try it, you might like it, certainly my wife loves it, but it's not to everyone's taste so just see if you like it and if you don't, don't be scared to say so. As jno said, sex should be about enjoyment, so don't be pressurised into doing anything you really don't want.
As said above there are so many different degrees of being tied up.
Have a chat first and get an idea of each other's idea of what you'd both want or not want to happen so there is less chance of either going beyond the other's boundaries. Communicate throughout, agree to both stop whenever either of you wants to.
Trust is a big issue too as although fun it can leave you very vulnerable. If it's a fetish of his a different (although not necessarily bad) side of him could emerge which could be a bit of a shock if you've not talked it through first.
I'd suggest going slow and gentle at first, make it fun and imimate, use silk scarves, they don't need to be tight enough to restrain, the idea can be enough.
Be very clear about what happens while you're tied up as well. Don't do it if you're unsure, stop if you need to if you do give it a go, be honest and don't just carry on because you think it's doing it for him.
If you're still not sure you could maybe get some videos of the kind of thing that he wants to do so you have a better idea of what he wants and whether it's what you want (or to that limit).
I'd also make it clear that any kind of photographic equipment is banned if you wouldn't be comfortable with that as the possibility is there to be taken advantage of.
Only if you want to and if you totally trust him. Agree beforehand on a word you will use to indicate you seriously want him to stop whatever he's doing and untie you. Choose a word you wouldn't normally use in this situation - like 'pickled onions!'