In a lot of cases, you can easily log in and log out (or create and destroy) with multiple user accounts in setup() and teardown(). This can be done by just having a higher setupTimeout / teardownTimeout values and/or making the HTTP requests with http.batch() and returning the resulting credentials in an array. Then each VUs can just pick its own credentials from that array by using the __VU execution context variable and some modulo arithmetic, somewhat like this:
export default function(setupData) {
let myCredentials = setupData[__VU % options.vus];
// ...
}
That said, the recently released k6 v0.27.0 also adds another option - the per-vu-iterations executor. It, and fact that you can have multiple sequential scenarios, combined with the property that k6 will reuse VUs between non-overlapping scenarios, means that you can make something like a per-VU initialization function! 
This can be done by just having a scenario that does 1 iteration per VU and persisting any data or credentials in the global scope. Here’s a very complicated example that demonstrates this workaround, and it even includes thresholds to make sure that the per-VU setup and teardown methods are executed correctly:
import http from 'k6/http';
import { sleep, check } from 'k6';
import { Counter } from 'k6/metrics';
import { uuidv4 } from 'https://jslib.k6.io/k6-utils/1.0.0/index.js';
let VUsCount = __ENV.VUS ? __ENV.VUS : 5;
const vuInitTimeoutSecs = 5; // adjust this if you have a longer init!
const loadTestDurationSecs = 30;
const loadTestGracefulStopSecs = 5;
let vuSetupsDone = new Counter('vu_setups_done');
export let options = {
scenarios: {
// This is the per-VU setup/init equivalent:
vu_setup: {
executor: 'per-vu-iterations',
vus: VUsCount,
iterations: 1,
maxDuration: `${vuInitTimeoutSecs}s`,
gracefulStop: '0s',
exec: 'vuSetup',
},
// You can have any type of executor here, or multiple ones, as long as
// the number of VUs is the pre-initialized VUsCount above.
my_api_test: {
executor: 'constant-arrival-rate',
startTime: `${vuInitTimeoutSecs}s`, // start only after the init is done
preAllocatedVUs: VUsCount,
rate: 5,
timeUnit: '1s',
duration: `${loadTestDurationSecs}s`,
gracefulStop: `${loadTestGracefulStopSecs}s`,
// Add extra tags to emitted metrics from this scenario. This way
// our thresholds below can only be for them. We can also filter by
// the `scenario:my_api_test` tag for that, but setting a custom tag
// here allows us to set common thresholds for multi-scenario tests.
tags: { type: 'loadtest' },
exec: 'apiTest',
},
// This is the per-VU teardown/cleanup equivalent:
vu_teardown: {
executor: 'per-vu-iterations',
startTime: `${vuInitTimeoutSecs + loadTestDurationSecs + loadTestGracefulStopSecs}s`,
vus: VUsCount,
iterations: 1,
maxDuration: `${vuInitTimeoutSecs}s`,
exec: 'vuTeardown',
},
},
thresholds: {
// Make sure all of the VUs finished their setup successfully, so we can
// ensure that the load test won't continue with broken VU "setup" data
'vu_setups_done': [{
threshold: `count==${VUsCount}`,
abortOnFail: true,
delayAbortEval: `${vuInitTimeoutSecs}s`,
}],
// Also make sure all of the VU teardown calls finished uninterrupted:
'iterations{scenario:vu_teardown}': [`count==${VUsCount}`],
// Ignore HTTP requests from the VU setup or teardown here
'http_req_duration{type:loadtest}': ['p(99)<300', 'p(99.9)<500', 'max<1000'],
},
summaryTrendStats: ['min', 'med', 'avg', 'p(90)', 'p(95)', 'p(99)', 'p(99.9)', 'max'],
};
let vuCrocName = uuidv4();
let httpReqParams = { headers: {} }; // token is set in init()
export function vuSetup() {
vuSetupsDone.add(0); // workaround for https://github.com/loadimpact/k6/issues/1346
let user = `croco${vuCrocName}`
let pass = `pass${__VU}`
let res = http.post('https://test-api.k6.io/user/register/', {
first_name: 'Crocodile',
last_name: vuCrocName,
username: user,
password: pass,
});
check(res, { 'Created user': (r) => r.status === 201 });
// Add some bogus wait time to see how VU setup "timeouts" are handled, and
// how these requests are not included in the http_req_duration threshold.
let randDelay = Math.floor(Math.random() * 4)
http.get(`https://httpbin.test.k6.io/delay/${randDelay}`);
let loginRes = http.post(`https://test-api.k6.io/auth/token/login/`, {
username: user,
password: pass
});
let vuAuthToken = loginRes.json('access');
if (check(vuAuthToken, { 'Logged in user': (t) => t !== '' })) {
console.log(`VU ${__VU} was logged in with username ${user} and token ${vuAuthToken}`);
// Set the data back in the global VU context:
httpReqParams.headers['Authorization'] = `Bearer ${vuAuthToken}`;
vuSetupsDone.add(1);
}
}
export function apiTest() {
const url = 'https://test-api.k6.io/my/crocodiles/';
const payload = {
name: `Name ${uuidv4()}`,
sex: 'M',
date_of_birth: '2001-01-01',
};
let newCrocResp = http.post(url, payload, httpReqParams);
if (check(newCrocResp, { 'Croc created correctly': (r) => r.status === 201 })) {
console.log(`[${__VU}] Created a new croc with id ${newCrocResp.json('id')}`);
}
let resp = http.get(url, httpReqParams);
if (resp.status == 200 && resp.json().length > 3) {
let data = resp.json();
if (data.length > 3) {
let id = data[0].id;
console.log(`[${__VU}] We have ${data.length} crocs, so deleting the oldest one ${id}`);
let r = http.del(`${url}/${id}/`, null, httpReqParams);
check(newCrocResp, { 'Croc deleted correctly': (r) => r.status === 201 })
}
}
}
export function vuTeardown() {
console.log(`VU ${__VU} (${vuCrocName}) is tearing itself down...`);
// In the real world, that will be actual clean up code and fancy error
// catching like in vuSetup() above. For the demo, you can increase the
// bogus wait time below to see how VU teardown "timeouts" are handled.
sleep(Math.random() * 5);
console.log(`VU ${__VU} (${vuCrocName}) was torn down!`);
}