It's still okay if it's one on one, but it's tough if there are several people in the meeting.
It's still alright if it's one on one, but it's difficult if there are multiple people participating in the meeting.
「一対一」→「One on one」
「まし」→「Okay, alright」
「複数」→「Several, multiple」
「参加する」→「To participate」
「きつい」→「Tough, difficult」
まとめて言いますと、
「It's still okay if it's one on one, but it's tough if there are several people in the meeting.」
「It's still alright if it's one on one, but it's difficult if there are multiple people participating in the meeting.」
と表現します。
どちらでも使えます。