If you are a tweetup group owner, use your tweetup group account if possible to follow @tweetup_. Then, direct message announcements (like "Sydney meetup this Sunday at 10pm. Pub at corner of 5th ave and 6th st."), they will be retweeted thanks to GroupTweet.
Once you follow me I will get the e-mail letting me know you have done so, will verify you're a group or group owner (please have something about it in your bio!), and follow you. This lets you direct message @tweetup_ announcements and announcements only please! Thanks!
P.S. remember to subtract the length of your name plus 7 characters from the length of your message. you can type "via @yourname: " at the start of the direct message box and delete it before sending to make sure you're in the limits.
If you'd like to follow the latest announcements from all tweetup groups about when the next tweetups are, follow @tweetup_. It relays announcements sent to it by group leaders, of which anyone can be one. If you want to announce a tweetup, follow the directions above for owners. If it's a one time tweetup, you can @reply tweetup or e-mail tweetup at (this domain name). Logic used to confuse spambots :)