Well, it depends on two things, Team Leaders and the individual member. In the past, some invited members have declined the invitation to assume the greater responsibility and commit themselves to the volunteer role.
I can’t speak for the Team Leaders, but I can offer some advice that could help.
Become a Trust Level 3 Regular member by reading many topics, visiting very often for a long time. make posts that others will Like and not Flag.
Help others to learn by sharing your knowledge with those that know less than you. You don’t need to be an expert in every area of web development, but no matter where you are on the scale for Design, PHP, JavaScript, Server Configuration, Databases, etc. (i.e. the various web dev categories of the forum) there will likely be members that could use some help from time to time just as at times we all could use help with something we’re not so knowledgeable in) For example, I might be able to solve a JavaScript problem but am totally stumped by a CSS problem.
Have a love of web development and the community that is the SitePoint forum. Be welcoming and friendly to others. Don’t try to “earn” becoming Staff and don’t expect to be invited regardless of how much you may deserve to be. There have been many members that IMHO would have been good as Staff, but were not invited simply because there was no need for another staff member.