To join you will need the FTB Legacy launcher found here
Or you can use the curse launcher found here
Click on Create Profile in the lower right hand (If not already done)
Click on the 3rd Party Packs tab and click on Regrowth
Install the pack (You don't need to add any additional mods)
Click Launch to launch Regrowth

We are currently running the version listed here

North America:
Server ip: reg.devcoftb.net