Learn Croatian: Okupit ćemo se oko stola. - We will gather around the table.

The preposition oko always requires the noun following it to be in the genitive case. Since stol (table) is a masculine noun ending in a consonant, its genitive form adds an -a, becoming stola.

Why is the verb written as okupit instead of okupiti?

When forming the future tense, if the infinitive ends in -ti and comes directly before the auxiliary verb (ćemo in this case), the final -i is dropped. So okupiti ćemo becomes okupit ćemo.

What is the purpose of se in this sentence?

The verb okupiti alone means to gather objects (like collecting documents). To say that people gather themselves together, you must use the reflexive form okupiti se.

Could we change the word order to "Okupit se ćemo oko stola"?

No, because the order of enclitics (short, unstressed words) is strictly fixed in Croatian. The future auxiliary verb ćemo must always come before the reflexive pronoun se.