May broke off from the UK election campaign to host a working dinner in Downing Street with Barnier and Juncker.
On Thursday, a Commission spokesperson said May restated the UK's commitment to a "deep and special partnership" with the EU post-Brexit.
The spokesperson said Juncker and May discussed the broader geopolitical agenda and issues of strategic interest to both the EU and UK.
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Britain is thought to be pressing for low-key technical talks, with discussions among officials, while the EU is looking for a more transparent approach.
The talks come ahead of a summit of EU leaders in Brussels on Saturday which will discuss the EU's negotiating guidelines and Brussels' opening position in the talks due to start in June.
Leaders are expected to demand that May respects the rights of EU nationals who have lived in the UK for five years to acquire permanent residence in the country.
On Wednesday, the UK Brexit secretary David Davies told business leaders he wants to secure rights of EU nationals living in Britain at the earliest opportunity.
"The government has made it very clear it wants to secure the rights of EU nationals living in Britain at the earliest chance in the negotiations," Davis said, who attended Wednesday's dinner with Juncker and Barnier.
Davis also said that the talks with Barnier and Juncker had taken place in an atmosphere of "mutual agreement" on the importance of the rights issue.
British officials have said details of the negotiating strategy and team were still being decided.
Negotiations between Britain and the EU are due to start after June's UK election, with the size of the so-called divorce bill facing Britain one of the first thorny items on the agenda.
Barnier has previously insisted the UK must "settle the accounts" before beginning talks on a future trading relationship with the EU.
Senior members of the British government have insisted Britain will not pay a bill of the scale suggested by the likes of Juncker, with international trade secretary Liam Fox branding the idea "absurd".
But May has said she is ready to discuss a "fair settlement" of Britain's obligations, although she has given no hint of the amount she is prepared to pay.
Britain and the EU have just two years in which to untangle their four-decade relationship and forge a new one.
Speaking earlier this week, UK foreign secretary Boris Johnson said he was confident the UK would get "a good deal that works for both Britain and our European friends".
Separately, a poll by Sky Data has found 55 per cent of respondents oppose a deal offering a unilateral guarantee for EU citizens living in the UK after Brexit, while only 29 per cent would support it.
Some 44 per cent said they would be in favour of a unilateral guarantee only if it was reciprocated by the EU for UK citizens living abroad. The poll also found more respondents assessed the government's performance on Brexit as bad (42 per cent) than good (25 per cent).