Joining an existing one means having to deal with existing members. If you want an organization to advance a certain set of goals, whether they're policy goals or just networking with similar people (given that similar(LW) is different than similar(freemason)), it seems like it'd be easier to have general agreement, shared background, etc. across all the members, which is something you don't get from entryism -- you have to expend energy on spreading that background, getting existing members to align with the entryists, and so on.

Admittedly, I also have aesthetic problems with going "your social club is now our rationality group", but my priors in the direction of freemasons-as-just-a-social-club are not all that strong, due to both little information and the geographical sources of that information -- for purely statistical reasons, I wouldn't expect many rationality groups way out in the hills.