An unconventional move indeed, but popular with ukippers who have succeeded in driving out or silencing many of the remain minded members and MPs.
The already eurosceptic membership has been flooded with ukippers, and these are the people who will be voting on the next tory leader (our next Prime Minister), something that should be filling us with dread, and possibly also deselecting the few remaining conservative MPs they see as ‘soft’.
If half those who went to the People’s Vote March joined (and they probably spent as much as the membership fee on travel to London), then there would be a remain majority in the conservative membership, maybe Hunt would have won the leadership contest instead of Johnson and we would have a softer brexit, or even a hung parliament and a new referendum.
If you’re concerned about your membership fee being used to promote brexit, then if you can afford it nullify this effect by donating the same as the membership fee to another organization that promotes re-join.
There are still conservatives who are pro-europe and they have an organization far older than the ERG: Conservative European Forum
What if I’m already a member of another party?
There is no law against being a member of more than one party. Each party has its own membership rules. You would have to be a high-profile individual to get spotted and excluded for also being in an opposition party: https://en.wikipedia.org/wiki/Political_party_affiliation_in_the_United_Kingdom The frequency with which the parties compare membership rolls is slightly rarer than hen’s teeth, so for most people this is a matter of personal conscience only.
Arguments against this action
Following on from the above point, mass joining the Conservative party is not endorsed by all re-join groups and commentators, some feel it is dishonest – though given the number of conservatives who joined Labour to elect Corbyn as leader, you could argue it is justified and will give them a taste of their own medicine. There are many less political individuals who just want to re-join the EU, don’t feel inspired by the other parties, and may have voted Conservative previously.
It has also been pointed out that the Conservative party is not as democratic as other parties, especially when it comes to leadership elections. Recall how there was no election for Teresa May in 2016, as all other contenders dropped out, and how the 2019 election was between two Brexit supporters (Rory Stewart had been eliminated in earlier rounds of voting by the parliamentary party).
Still, getting more remainers in the Conservative party can’t be a bad thing, and may drive the hard brexiteers away to UKIP and other extremist parties, and also embolden pro-rejoin Conservative MPs to speak up without fear of their local members.
Things you can do as a member
You could just sign-up on the website, then filter all their emails to a special folder, and forget you were there until the next leadership election or renewal time.
Or you could actively participate in their online members forum, go to local meetings, and the annual conference and try to brexorcise other members!