Is it legal if they have only paid me about 20% of the agreed sum for my shares, but they are selling shares to other people?

If they owe you 80% and haven't paid, you can sue for the 80%. What does their selling share to somebody else have to do with it?

The question of "legality," in terms of whether or not some law is being violated, cannot be answered at this time. There is simply not enough information. If you have a contract for the repurchase of your interest in the LLC, however, then you may have a claim for breach of that contract. This claim exists even if no laws are broken. You should contact counsel at your earliest convenience to determine how best to enforce your contract, and to see if you have any additional claims against the remaining principals in the LLC.