There are two numbers some folks are interested in:
1) Total Membership Accounts
2) Active Membership (the important one)
For #1, Total Accounts (which includes inactive and active accounts), scroll to near the bottom of the main page and look in the stats for this line:
Front Range 4x4 Forums Statistics
Threads 12,608 Posts 196,971 Members 605
As of this post, total membership (active + inactive) is 605.
For #2, Visit the Member List screen, Click "Search Members" > "Advanced Search". Leave all of the search boxes empty and click the "Search Now" button. Scroll to the bottom of any search results screen and look for this line: "Results 1 to 50 of 240". The 240 in that line is how many active member accounts are on the forum.