On the agenda at Constituency Labour Party meetings should be a list of new members. The idea is that the GC can use this to spot entryists from other parties, people that might bring the Party into disrepute etc. and flag them up to the NEC.
Back in the '80s when I joined you even used to have a special "provisional membership" card signed by your CLP secretary (with the old Clause IV on the back) until the GC had endorsed you (like candidate membership of the CPSU). My GC in Canterbury was rather leftwing and used to turn people down for not being in a union etc. - in one case rejecting an application from an MEP's researcher as he was a member of the Syndicat des Travailleurs Belgiques (Brussels Euro-Parliament Branch) and this wasn't a TUC affiliate ...
Anyway, I hope that the GC in the CLP where Guardian columnist John Harris has re-joined will have a careful think about whether we really want him.
Of particular relevance was his book at the time of the last election which advocated voting for the Lib Dems, SNP, Plaid, Respect and the Greens in various seats where the MPs did not meet his ideological criteria: http://books.guardian.co.uk/extracts/story/0,,1385618,00.html
Isn't this exactly what Clare Short is facing disciplinary action for?
It would also be interesting to know how Harris came to be billed as a "Compass stalwart" by the Guardian some six months before he had even applied to be a member of the Labour Party.