Any propane setup has a couple items to keep in mind on the reg. Each line is high pressure before the reg and low after, so you want one and only want one reg cutting the pressure for any specific item. So if you put a tee before the existing reg, it will be feeding high pressure to the new connection and that means some form of reg there. If you add a tee after the reg, then no reg at the new point. So think of what grill, etc is used at the end and set up to match the item used. Some grills have a reg onboard while others do not. Just knock the pressure down one time or it is likely to not be enough pressure to let the grill work very well.
If adding the connection to let you use gas from a portable tank instead of the permanent, add the tee before the reg and turn one tank on at a time to avoid draining both but the single reg is all that is needed.