How to Use USA Softball Register Play
Visit registration page:
https://ma.registerplay.com/Click the Sign-In link in the top right corner; create a new account. (I had an account when it was previously registering with USA softball, but had to create a new one for USA Softball of Massachusetts):
Note: After creating a new account, you may have to click Sign In and enter your username/password again to get signed in.
Go to the Dashboard tab; if your child is not shown click Add Your Child:

Fill in details for your child and click Add Child:

Your child should now show under the family members; click Join a Team:

Fill in the with the information from your roster invitation including the password and click Join:

Confirm the team, read the waiver, and click Confirm & Join:

Your player should now show as rostered!