You're talking about a pretty major group ride, and it will be normal for groups to link up at various stages during it. Usually a "Howdee... mind if we tag on behind?" will do the trick, but you should really ride up alongside to ask... a yell from behind, especially if you have crept up without gearchanges and stuff, or the riders are deep in conversation or concentration can frighten the living daylights out of them.
As mentioned by others, your speed might be the same, but their skills might not match yours. Things like standing to pedal in a pack to relieve the butt can cause serious problems if a rider doesn't get it right (gearshift down and stand while maintaining exactly the same speed is an art). Even retrieving and replacing a water bottle will give a clue to group riding skills.
It might be worth discussing about pulling, too. One rotation will pretty quickly determine if there are any skill deficiencies in this area... if there is a trend for the new front rider to speed up at all, forget it. Communication with hand signals or by voice about obstacles ahead, or changes in direction and speed, is vitally important.
The most important thing to remember if you do tag on is that *you* will be the one who goes down if someone up front makes a bad move. So you will need to concentrate very hard on wheel-to-wheel distance and avoid overlap until you are satisfied you can be comfortable with the group you have joined. If you aren't comfortable, pass, or drop back a bit.