Before implementing Blackbaud, we used Edmodo or Google Classroom for our clubs. The student was given a code, which they typed in and were automatically added to the roster. The amount of work our teachers/activity manager has to go through to add students to clubs is too much, and very tedious.
This could be done by making Activities operate like the Community Group type of 'Listed Anyone', which can be joined under 'Classes> View Other Classes'