Default user role

Hello,

When user’s accept an invitation and register, they are assigned the role of “participant.” This causes them to get an error message, when they try to log in again:

“You do not have sufficient permissions to access this page.”

If I change their user role administratively to include one of the default WordPress roles, like subscriber, they no longer get this error.

Is there a way to have users assigned the default site user role when they accept a registration invitation (or some other recommendation)?

Thanks!