Many of the regranting organisations were/are largely volunteer run. EAA only hired a part time staff member to do accounting after ~4 years running and moving $1m+ per year (now at ~$4 million p/a with still <1 FTE).

Pretty much every major EA org I can think of started volunteer run while people were working/studying. CEA started on the back of Giving What We Can & 80,000 Hours which had been mostly volunteer run by students/academics.

Not sure if this fits, but it seems like 80,000 hours started as somewhat of a side project. This 2015 article it says 80k started with Will MacAskill and Ben Todd “forming a discussion group and giving lectures on the topic, then eventually creating 80,000 Hours to spread their ideas.” (They link to this vintage lecture they gave.)

I’m not sure how much of a “side project” this was to Ben and Will. Maybe others know more about that era.