Learn Portuguese (Portugal): Eles podem teletransportar-se, além de serem invulneráveis. - They can teleport, in addition to being invulnerable.

The phrase além de (in addition to) is a preposition. When a verb follows a preposition and has a clear subject (in this case, Eles), European Portuguese uses the personal infinitive. Because the subject is they, the infinitive ser takes the third-person plural ending to become serem.

Is there a difference between além de and para além de in this sentence?

No, they mean exactly the same thing here. You could easily say para além de serem invulneráveis and it would still mean exactly the same thing. Both require the personal infinitive.

Could the reflexive pronoun in teletransportar-se be placed somewhere else?

Yes. In verb phrases with an auxiliary verb (podem) and an infinitive (teletransportar), you can attach the pronoun to the end of the infinitive, as seen here. Alternatively, you can attach it to the auxiliary verb: Eles podem-se teletransportar. Both are perfectly natural in Portugal.

Why does invulneráveis end in -eis?