Two ways to do this:
1) cut out about 3/4" - 1" of pipe. Loosen a couple supporting straps. You should have enough 'play' to be able to bend one pipe , slide the tee on, and then jimmyshimmy the other pipe end onto the tee.
2) Cut out a 1' section of the pipe. Solder the tee to one end of the cut out section. Cut the other end of the end of the cut out section about 1/2 the length of the tee. You want the tee-pipe to almost completely make up the gap between the sections of the main line after you slide the tee back onto one section. Then you use hubless coupler to connect the cutout to the other end of the main line. The hubless doesn't have a ridge in the center. This means you can slide it all the way onto the main line, position your cutout into place and slide the coupler back over the cutout. No jimmyshimmy necessary. The downside of the hubless is that you gotta be careful about sliding the coupler 1/2 way over both sections. The hubbed coupler takes the guesswk out of that part, but some jimmyshimmy will be required.
IMHO, though, fitting the tee's the easy part. Getting the water out so you can solder is the PITA part