First I will say that you don't want to send it to every everyone, i.e. landlines, contacts that only have email addresses etc. My guess is that you have a target list of people you want to reach.
Second let me say that creating large group messages, 20 or more contacts, often fail to reach everyone. I've seen them fail at one number and that cause some to not receive the message at all.
My best advice is to open your SMS app create the message and copying that message to your clipboard, which is simple. Highlight>Copy. Then add people from your contacts, starting at the top and working your way down, selecting 10 to 12 contacts at a time. So you will likely send out 8 - 10 messages depending on how many friends you have an how large you make your groups, however by simply pasting the same message in each text it really won't be that much work.
Oh and it gives you two opportunities that you wouldn't get trying to carpet your contacts list. First by looking at each contact and deciding to add them will prevent you from accidentally sending to anyone you may have saved that you don't want to have the number, i.e. Ex's, people who may have caused you to change your number in the first place, and people you just don't talk to anymore. The second thing it will allow you to do is take a serious look at the contacts you have saved in your phone. Allowing you to get rid of numbers you just don't ever call, will never call or ones that you don't even remember the person. That person you met that one night at that one place where you were exchanging numbers just to be friendly and gave them your real number instead of your friend's... Don't look at me like that you know what I mean.