Not all spider/bots are recognized as such by the SMF software, so may show as 'guest' instead.
Also, we have a pot-load of miscreant bots attempting registrations so they can sign in and spam us.. Blocked by our trusty dual captcha system. Here's yesterday's registration attempts 397 109.123.118.8 UK2 - Ltd, United Kingdom
150 178.63.60.23 Hetzner Online AG, Germany
124 80.95.44.222 MRF-VOLGA-PENZA-DYNPOOL-25-03-2013-2, NAT Pool Penza, RU (Russia)
78 77.83.174.247 NET-100-2, NL (Victoria, Mahe, Seychelles)
72 120.14.140.34 China Unicom Heibei Province Network, China
67 77.37.226.109 OJSC Rostelecom, Russia
62 95.84.56.162 OJSC Rostelecom, Russia
62 195.39.196.253 EU-ZZ-193-194-195, Europe, EU (The Netherlands)
57 93.120.156.67 DYNAMIC-BRAS-POOL5-NNOVVT, NMTS Autonomous System, RU (Russia)
57 46.188.98.10 2COM Co ltd., Moscow, Russia
57 31.13.190.254 M247-LTD-Frankfurt, M247 LTD Frankfurt Infrastructure, DE (60314, Frankfurt, Germany)
57 195.206.105.217 EU-ZZ-193-194-195, Europe, EU (The Netherlands)
57 194.1.238.114 EU-ZZ-193-194-195, Europe, EU (The Netherlands)
52 85.209.90.153 NET-108-2, NL (Victoria, Mahe, Seychelles)
51 213.178.45.41 RU-AIST-20080117, AIST Networks, RU (Russia)
50 185.136.166.126 VELIANET-FR-YURI, velia.net Internetdienste GmbH, FR (Russian Federation)
...
When they get really obnoxious, I add them to the .htaccess deny list (usually an entire CIDR worth) to give them 403s instead.