The question is quite broad. If you're new to HackerNews check https://github.com/cjbarber/ToolsOfTheTrade Many services are SaaS with monthly pricing and there's a lot of promotion (startups adding themselves). The HR section seems relevant.

I'd say the majority starts simply to lots of Excel/GoogleDocs spreadsheets, then add chat tools for the website, then messaging tools for the team to manage one mailbox (https://frontapp.com/, https://missiveapp.com/ and such), possibly https://calendly.com/ to let user add themselves to calendars, sales tools like https://close.com/

High growth might then require consolidated data with tools like Salesforge, https://www.hubspot.com/, https://segment.com/ To integrate and finetune those can be a full-time (engineering) job.