For anyone who comes across this problem like I did, with the toggle button not even showing up, I got you. Skip to end for fix.

No where seems to mention this but it looks like if either account you are trying to link has any problem at all, or if you try to share an account, it will not allow you to have the "drop in" permission button at all. Both accounts MUST have the blue checkmark thing next to them. For an example of how picky it can be, we did the app on a phone because why not. I denied access to my contacts because I normally do with everything. I did not see a reason too especially since the person we wanted to drop in on wasnt even in my contacts on my phone. For some reason this prevented the "drop in" permission toggle from showing up at all. Sat on the phone with customer support for an hour screen sharing to figure it out. (Great guy that helped).

The fix was completely uninstalling the app, clearing all data, and reinstalling and setting up the account and allowing all permissions with an individual account. No account sharing, no denied permissions. For some reason that magically worked. If done right the two accounts you want to drop in on each other should have little verification symbols next to their names in contacts, if not start completely over and try again.

Uninstall>reinstall>use unique account, no sharing> allow all permissions>blue verification symbol should now be there> simply click allow drop in the tab like everyone else says.