I'm trying to figure out how to email all users directly from LimeSurvey to invite them to an OPEN survey (in other words, they can in turn invite others, etc to answer the survey, so it's wide open, BUT i still want to invite my potential respondants in LW).

Found it... go to Configuration > Create/Edit user groups>
then click on desired group
then "Mail to all users" button...
fill out form... boom

Just to make sure:

The "users" in Limesurvey are generally survey admins, not respondents. Of course they can respond surveys.

But you don't want to make "normal" respondents survey admins (limesurvey users). Because they can edit the surveys that are connected to a certain user group.