130 of the eighth and current European Parliament's 751 members have signed a letter urging the UK to reconsider its decision to withdraw from the EU. All are members of EPP, S&D, ALDE or Greens-EFA, which together comprise 525 of the members. (All statistics based on this table, which gives the groups' respective sizes as 217, 189, 68 and 51.) None were drawn from the other five political groups of the current Parliament - the ECR (74 members), GUE-NGL (52), EFDD ((45), ENF (37) or Non-Iscrits (18). If all groups' members were equally likely to sign the letter, the Non-Iscrits would be expected to include 3 signatories, and the ECR 13. Therefore, it's natural to assume the political views of these groups at least partly explain who signed. But how?
I'll paraphrase the groups' Wikipedia pages' summaries of their composition.
- Partially signatory groups The EPP is a coalition of centre-right, Christian democratic, conservative and liberal-conservative parties; S&D comprises socialist, social-democratic and progressive ones; ALDE is liberal-centrist, with liberal and democrat members; and Greens/EFA combines green, regionalist and left-wing nationalist parties.
- Non-signatory groups The ECR is Eurosceptic and anti-federalist, with conservative and reformist members; it is right to centre-right. GUE/NGL is left-wing, containing socialist and communist parties. EFDD is populist Eurosceptic. ENF comprises right and far-right parries, including France's National Rally, which is right-wing and populist, and its and its members' references to "freedom" advocacy reflect Euroscepticism. The Non-Iscrits have a varied historical composition, but since 2015 have comprised mostly far-right MEPs, several of them former EFDD members (although Alessandro Mussolini recently left the EPP).
In other words, Eurosceptics unsurprisingly don't urge the UK to stay in the EU; if anything, they might hope their own nations will do the same in due course. Meanwhile, signatory groups are very varied in their policies, and any one of them is too; but you might say they represent the "default" MEP style, which ought to support the EU, otherwise why sit in its Parliament?
The only real outlier among the nine groups is GUE/NGL, which would be expected to include 9 signatories. So why don't they? I'm not an expert on European politics, so I don't know why socialists or communists would be unwilling to advise the UK to stay in the EU. (For all I know, it's not so much they think leaving the EU is prudent as that they don't want to tell others what to do, or that they think getting involved in the dispute is against their interests.) Of course, it might not be as simple a matter as saying "neither Communists nor socialists would sign because X", because socialists are found in the S&D. (It's possible none of the S&D signatories are socialist specifically; I haven't checked.) But in broad terms, the GUE/NGL's absence from the letter is surprising, because the signatory groups are mostly to the left of the non-signatory ones.