Go on the Sirrus! If it's a good club there will be no problem.
All my life I've been a dedicated roadie but I see people come to our clubruns on all kinds of bikes. They're always welcomed and accommodated. In truth, very few have to be waited for (and as has been noted there's always a lanterne rouge encouraging back markers). TO me, if you're prepared to try a club that's brilliant - it's another cyclist on our roads.
Ride with the club, I'm almost certain you'll start to pine for a 'road' bike pretty soon after joining but isn't that half of cycling anyway? n+1 and a bit of peer pressure?
If it's a good club you'll have revolutionised your weekends for life!