Can a juvenile be bailed out by an adult beside the parents or guardian?

Good question.  There are a couple of laws on bail.  In terms of local rules of the court, contact your counsel.  In terms of other laws, the bail by someone who is not a guardian or parent may not be accepted because of possible harboring of minors/contributing to the delinquency of minors laws.

Section 63. Any person who shall be found to have caused, induced, abetted, or encouraged or contributed toward the delinquency of a child, or to have acted in any way tending to cause or induce such delinquency, may be punished by a fine of not more than five hundred dollars or by imprisonment of not more than one year, or both. The court may release on probation under section eighty-seven of chapter two hundred and seventy-six, subject to such orders as it may make as to future conduct tending to cause, induce or contribute to such delinquency, or it may suspend sentence under section one of chapter two hundred and seventy-nine, or before trial, with the defendant’s consent, it may allow the defendant to enter into a recognizance, in such penal sum as the court may fix, conditioned to comply with such terms as the court may order for the promotion of the future welfare of the child, and the said case may then be placed on file. The provisions for recognizance in section fifty-six shall be applicable to cases arising hereunder. The divisions of the juvenile court department shall, within their respective territorial limits, have exclusive jurisdiction over complaints alleging violations of this section.

Here are the laws on bail:

Section 74. Except as hereinafter provided and as provided in sections fifty-two to eighty-four, inclusive, no criminal proceeding shall be begun against any person who prior to his seventeenth birthday commits an offense against the laws of the commonwealth or who violates any city ordinance or town by-law, provided, however, that a criminal complaint alleging violation of any city ordinance or town by-law regulating the operation of motor vehicles, which is not capable of being judicially heard and determined as a civil motor vehicle infraction pursuant to the provisions of chapter ninety C may issue against a child between sixteen and seventeen years of age without first proceeding against him as a delinquent child.

The juvenile court shall not have jurisdiction over a person who had at the time of the offense attained the age of fourteen but not yet attained the age of seventeen who is charged with committing murder in the first or second degree. Complaints and indictments brought against persons for such offenses, and for other criminal offenses properly joined under Massachusetts Rules of Criminal Procedure 9 (a) (1), shall be brought in accordance with the usual course and manner of criminal proceedings.

Section 60. After a conviction or a plea of guilty or of nolo contendere in the superior court in Suffolk county, the prisoner shall not be admitted to bail except in open court; but when said court is not in session, bail may be taken by any judge of a court of record or by any commissioner appointed under section fifty-seven, upon proof that written notice of the proposed application has been duly served upon the district attorney, or one of the assistant district attorneys for the Suffolk district, at least twenty-four hours before the hearing of such application, specifying the name of the prisoner, the crime of which he has been convicted, the time and place of hearing, and the name, occupation and residence of the proposed sureties. No person who has been once offered and rejected as surety shall afterward be accepted as surety for the same person in the same case.